As you say you already have FH3, I don’t quite understand the question in the thread title. I’m presuming this is one of those loaded questions for people to praise FH3 and hate on FH4.
Like you’ve already pointed out, you can try it out on game pass. If you like it the dlc is more of the same. If do like the game the dlc is the price of a decent meal, so I would say it’s good value.
FH4 is a good beautiful game but online is crap. Even in singleplayer mode drivatars are so aggresive and always want to ram you if you are close to them. They don’t care about slow down if you are in front of them or they can bump you from the side if you trying to pass by. It’s not every time but a common behavior. In FH3 I never saw this. But all and all FH4 is a good game if you don’t care much about MP and competing with other players.
Map is more interesting in FH4 but music is a lot better in FH3. But it’s a matter of taste.
Honestly? Neither. Forza Horizon 3 was worst game to come out with the Forza name on it imo. Forza Horizon 4 isn’t polished enough you recommend yet either. Wait 3 to 6 months on Horizon 4, then maybe purchase it.
There is benefits of owning many Forza games if you use Forza Hub.
I start Forza 5,6,7 and Forza Horizon 2,3 and 4 every friday to get 300.000 in each game, and i usually do Forzathons in Forza Horizon 3, 4 and Forza 7 weekly to get more money, free cars etc.
I’m Tier 10 in Forza Hub which pays 300.000 in each game. Outside Friday’s, i mainly play Forza Horizon 4.
I have all Forza games ever except for Forza 1. 
There is also smaller payouts weekly for Forza Horizon 1 and Forza 2,3,4.
For unknown reasons, i lost the saves in both Forza 3 and Forza Horizon 1 as late as year 2018, so i don’t play them anymore as it takes 100’s of hours, and i had loads of great auction cars that is impossible to get back. 
Wouldn’t help anything if the auction house were still online.
It sucks when savegames isn’t always safe with backup in cloud. (Xbox 360)
Xbox One cloud save have been 100% safe for me for years.
Something got messed up because of cloud sync on both games.
Anyway…I wonder why Forza 3 and 4 still isn’t backwards compatible…
I would wait. Fh4 at this time is less of a game than fh3. I would absolutely not paid 100+$ for the dlc edition. If you have access to the games pass try it out and dive into the online multiplayer areas. The single player content seems rather short to me, and the online content is straight up terrible imo. The online adventure mode is restricted to A,S1,S2(usually s1,s2) and you dont get pick the race type or season. And there is a freeroam sprint between every race pushing you to pick an offroad capable car even if you’re doing an S2 street adventure.
The coop/pvp races you can make will never attract more than 2 or 3 players so that’s DOA. And 4 months after launch all we’ve got to address these issues is the option to not be on a team (red v blue) during said adventure. And you cant play offline adventure, so you’ll spend several minutes matchmaking, queuing, and waiting for the group to finish a race you started in the middle of.
If your fine with these issues, then go ahead. I personally feel the game was shoved out the door too soon.
I’d jump in now, even though the online has issues, it’s likely to improve and as ever with online, if you join later in the games life everyone else has optimised cars and learned the track so online is way tougher
As above, you can always start with gamepass or look for a deal. Considering the hundreds of hours you’ll likely get out of it, it ok value
disclaimer - just my personal opinion
Fh4 doesn’t do much to improve upon the past game - if anything, fh3 is more complete, polished and generally more fun to play. you might find the different locale a refreshing change. but i don’t know, your call.
IMO FH4 is the superior game in many ways. There are more options to build your car (vintage tires for old cars, reintroduces fan-favorite engine swaps, track width upgrades, more bodykits) and the map is generally much better. FH3’s PI balance is also the worst among the three Xbox One games, heavily penalizing players for running better tire compounds.
FH4 also has “drag racing” (not a realistic mode but you can run it offline as opposed to in FH3), interesting cars absent from FH3 (M5 E60 for example), better graphics (there’s a noticeable improvement in lighting, in fact), Performance Mode on the Xbox One X if you’re interested in running the game at 60 fps and a more varied metagame when it comes to competition (FH3 strictly follows the AWD turbo V12 trope).
With regards to car sounds, both games compete over who will tarnish the franchise’s legacy the most, with both having pretty bad cars when it comes to sounds, but there are a few great-sounding cars in both (usually not the same ones across both games).
It’s hard to recommend someone over the internet a purchase like that, I really have no idea but I’d like to point out that as far as I know you can play FH4 via Gamepass and just get the DLC. I believe you get a discount from the DLC if you have the base game via game pass.
I’m a big fan of FH3 but personally I think FH4 is an improvement in nearly every single way, most importantly the driving experience and track design. Absolutely love FH4 and it was worth every penny for me and I’ve got hundreds of hours in FH3.
I uninstalled FH4 and went back to FH3, started over, and have been having fun ever since. I highly doubt I’ll be reinstalliing FH4 anytime soon, if ever. Don’t get me wrong, I like FH4, but after a certain point in the game, there’s nothing to do, that and the rubber-banding AI and traffic is just more than I’m willing to take.
I don’t know what it is about FH4… I enjoy it, but I’m not as invested as FH3. I found myself not really caring about certain types of races until just now for achievements. Where in FH3 I wanted to play all the race types.
With that being said, I’m a huge fan of Cross Country and I think FH3 did it better. I felt like I could take any car I wanted to and do so upgrades and be able to use it in a race. Now I feel like you have to take certain cars only, and even those suck. Like when you land a jump and your car cant seem to land well but the AI can?
With FH3 I think it was the fact to get more races and everything you had to explore, where now it’s just complete a few races and more pop up. I get that is the premise behind FH3… but in 3 there was a point to doing races to level up sites to get more races. Unlike 4 where after a few races I was bored.
I also fell like FH3 had more variety in terms of the map. Want to be in a city? check. have fun offroads in big open hill? check. Woods? check. FH4 there is no real “city racing” unless we talk about the small section at the top of the map where you can’t really open up and go for it in S2 or X.
On the flipside… seasons was a great addition.
and yeah
Personally I prefer 4, the map is better, the color palette has more than just brown and dark green, and the tracks are more technical, the only thing I dont like is team adventure but again online on fh3 was not much better, winning by drifting everywhere? really? Oh and freeroam rush also counted towards the score in 3, Anyway I dont care about avatars or clothes so I am neutral on that matter
I have played fh1, 2, and 3. No intention of getting 4 anytime soon. And I have so many reasons why but one that stood out was that they fixed what was missing in 1 and 2 with 3. And I didn’t see much of a difference between 4 and 3 minus the emotes and sh*t but let’s be honest who really looks at that apart from player level really.
Despite FH3’s problems with PI balance, it was a game that kept me playing for a full year.
FH4 has a good map, and some neat features like seasons, but when all the events are completed it becomes a car collection grind. I’ve given that up because I don’t enjoy any aspect of MP, so now visit the game briefly once a week.
FH4 is the first Forza game to launch without class based event leaderboards and I’m afraid that without them there is no motivation for me to build and race the cars I’ve collected. A total miss as far as I’m concened and I won’t be pre-ordering FH5
didn’t fm7 not have class based leaderboards at release but were released at least 6 months later
they homologated ones though for every track
They had plenty of leaderboards, which is the problem with FH4.
H3 is easily the better game overall. Much more polished. Doesn’t crash nearly as much H4. Most tracks are wider giving you more room to pass. Car sound is better. MP is way way better. Oh and what is the other thing that I’m missing??? You know where you could test ANY CAR and compete for times on an online leaderboard…what was that called? Oh yeah! RIVALS!!! Remember open class rivals!!!
H4 isn’t a total failure but the MP is the worst I’ve ever played in any racing game. Removing the points system equals win by all means necessary which equals bashing, ramming and wall riding. No open class rivals is the 2nd worst decision in the franchise. Car sounds are terrible. Game crashes way too often for a AAA game.
Ignore FH4 like the plague and just stick with FH3. Even FM5 is a better, more complete game than FH4 is and FM5 is easily the worst FM game.
FH3 is definitely the superior game. Much more race options. I don’t even have a favorite race track in FH4. FH3 is the AWD Meta but still much more fun. The soundtrack is better too.
give FH4 a miss.
The fundamentals have been broken from the start and with PG deciding to pile more stuff into a busted framework making it even worse now than it originally was . It’s too late to repair the game and the perception of the game.
The reputation of Playground Games has taken a hit also but I’d like to think that their pedigree can save them. Forza Horizon 5 will decide