If you have never played the previous 3 then I would recommend it highly. They are all great games but they get old really fast. There really isn't that many changes from FNR3 to 4.
i loved fight night round 3, sitting around with a friend smoking a blunt playing that game is alot of fun normally i don't even like playing games anymore but that has a sort of realism to it that just makes it abuncha fun. I'm sure 4 isn't much different.