No, there's nothing in 3 that isn't in 4 from what I can tell apart from some events. 4 contains most, if not all, of 3's DLC cars (or the ones that matter, anyway) and yeah, there's no need now.
Actually if really love your Japanese tuner cars, most of the ones from Forza 3 are now unicorns (i.e. unavailable to buy and very difficult to get hold of), like the HKS CT230R for instance. That annoyed me but not enough to make me not play.
These are the Japanese unicorns:
2004 Honda Mugen Civic Type-R
2002 Honda Mugen Integra Type-R
2005 Honda NSX-R GT
2002 Mazda RX-7 Spirit R Type-A
2006 Mitsubishi HKS Time Attack Evolution CT230R
1993 Nissan MINE'S R32 Skyline GT-R
2002 Nissan MINE'S R34 Skyline GT-R
2004 Nissan Top Secret Silvia D1-Spec S15
2006 Subaru Impreza S204
2002 Toyota Top Secret 0-300 Supra.
Madness, right?