Basically from what I have played, I will try to give a quick summary…I should say I am very pleased with how online works because there is a lot of variety and different ways to play online.
-You can Freeroam online(72 players max I think per server) and this is called being in a “Horizon Life” Session. Other people will be ghosted. You can quickly check if you are online by pressing Start/Menu, and if you see the message on the right side “Play Online”, you know that you are not yet in an online session. Usually it will join a server by default after a few moments, so you don’t really need to do anything. If you approach a ghosted player, you can interact with them and challenge them to a head to head race, invite them to your convoy.
-Whether you are online or offline, you can always press Start/Menu, and select Team Adventure, then choose Quickplay Adventure……this is unranked play, best 3 out of 5 events wins(team play), does game-wide matchmaking……usually very quick to get into one of those. In this mode, the seasons may change in between Matches(Same thing with ranked I think). Otherwise, most of the other modes are dependant on what season it is now in the servers, now it is Winter.
-From Team Adventure, you can also play Ranked at any time. You have to go through 10 matches first(qualifiers) before getting your ranking. It takes longer to get into this, but what I do is play around the map while it keeps trying to find a match and until everyone accepts…usually I get in a match within 10 min…best 3 out of 5 wins(team play)
-While Online In a Horizon Life session, you can stop at any of the colored event icons on the map(road racing, dirt racing, etc). From there, you can choose to play Co-op or PVP. Co-op is against Drivatars(team play). PVP is against other players and follows the traditional scoring method, not team play basically…whoever makes it 1st wins. Choosing one of those will initiate a 45 second timer and make a new icon appear on the left side of the screen of every other player who is in that server with you, the number 1 will be displayed and below it a prompt to press the A button(edit: may be different button for you depending on controller scheme). This will bring up the menu where the player can see the type of game you are hosting, and they can join you. For this method to work best, it helps to be in a server with a lot of players, like 40 or more. Usually I get 2-3 people joining my games.
-Seasonal Events: They change every Thursday. You can view those by going into your map and only show their icons in the filter. Also the welcome page has the Seasonal map info there as well. Usually, you have several regular icons that look similar(with the flag, or the road, etc) and one that looks different(Lion face). The regular ones can be played co-op or PVP. If you choose Co-op, it will work the same way I described above. If you choose PVP, it works differently, it will initiate game-wide matchmaking(after the current race ends) and will look for players in FH4 who are looking to race there, not just people in your current map server. This will initiate a championship of several events, and you race online against others with the traditional scoring method of getting points as you finish in higher positions.
The Seasonal Icon with the Lion Face works differently as far as its co-op is concerned, choosing that will initiate game-wide matchmaking as well, and it will be you and a bunch of other players against a team of highly skilled Drivatars. I quite like this mode, and as far as I know it is the only one that does matchmaking like that. Team play (Best 2 out of 3 )
Anyways, hope that helps some people, and I wrote this from memory a little quickly so apologies if there are mistakes and feel free to correct me there, but that is based on my experiences so far, I still have not tried many other features. Loving this game keep up the good work PG!