From the friend's list activity I've seen, people are only occasionally on FM4 - I see it come up say once of twice a week. That said, FM5 isn't a lot more active with generally 0-3 friends on at any one time.
Not really sure what to recommend if you're after multiplayer Forza - the new generation pretty much seems to have split the numbers on each version to down below critical mass.
Clubman GT Winner (only 'cos Jamesy got pished)
We all learn from our mistakes. I am very well educated...
RIP Stuart (Hoonta). Gone. Not Forgotten.