It'll obviously have more cars. That's going to include race cars, they aren't going to change their "winning formula" - Porsche will most likely not be included, it's just obvious, because EA have the rights. The physics of FM4 were rather good, much better than FH. And the DLC will be just as half-arsed as ever and massively overpriced.
I feel that FM5 will be next-gen and will be released in October. And it's clear that I'll probably buy it, along with a new console. I've not put any time into FH really, because it's just not that fun. I still get on FM4 and actually race, because that's what I'm interested in, because it's what I'm good at.
I'll always be a fan of the franchise.. and I'll always keep up with tech, because old tech goes out of sync very quickly these days it seems. I think it'd be a bad decision to release FM5 at the death of the 360, unless Microsoft doesn't plan on releasing the 720 until late 2014, which is a complete possibility.