I’ve been informed that its still possible on FM3, but would much prefer the later game if possible.
I know of the unicorn achievement which could cause a problem, but are there any more? Tried checking achievement sites but due to the age of the game you don’t get much response. Also I heard that DLC was removed from FM3, is this the case for FM4 as there was also DLC achievements?
fm3 has the ultimate version which has all the dlc included. i think the fm4 achievements were with the porche dlc and i’m pretty sure you can still buy that. there are achievements then there are titles and badges and i’m not sure if its just a title and badge or an achievement. there are a few like beat a developer which i haven’t seen any playing fm4 for quite a while. i do know to avoid the “essentials” version, its only half the game, its and big demo basically. you only get one of the two discs.
I have the Ultimate collection (F 3) but I get it 10 months ago and the problem is that this version of F3 is only 1 disc included + codes for DLC cars ( today is useless because the F3 servers are down)!!! If you want to unlock the achievements-you need the 2discs version of ultimate collection F3!!! Now about the F4- I have opened all the achievements and you really need the Porsche pack to unlock some of the achievements -hope that’s helps -if you have more questions-send me a message and I’ll try to help you-good luck- So2. 
It actually depends on where in the world you are as some places had a 2 disc version of the ulitmate edition. Still have to download the last car pack though. Also the Fm3 servers are still up - the DLC is just no longer in the marketplace.
I suppose if I wanted the 100% on Forza 4 I’d need to have Forza 3 anyway 
I might stick with getting a copy of Forza 3 for now. I did love the series, but the achievements got a bit silly. Forza 2 had a great list (if not exciting), Forza 3 was long winded after a while but still fun. Forza 4 I only played when I had no interest in achievements. I think what I may do is buy Forza 3 for my completion profile and then Forza 4 for my mucking about profile. That way I get the best of both worlds I suppose.
Its the problem with online achievements on any game. It can affect sales, especially digital which Ms want to push for. Where as if its got only single player stuff then you could sell the game still at full price a year down the line, as players would have full access to 100% of the attainable list. I think publishers try to push achievements to further sales and online use, but I don’t think they go about it the correct way.