If you really enjoy driving games like most everyone who dares post on these forums, then IMO you shouldn't pass up FM4. It is a more than a full version upgrade from FM3 and is absolutely in the top echelon of console drivers/racers. The physics and handling are far superior to FM3, which to me is reason alone. If you're into racers and you are also an F1 fan, F1 2011 should be in your collection as well (most probably the best of current gen console F1 racing titles.)
I ask cause i had a habit of buying racing games i didnt play much. mainly cause the wait for gt5 was driving me nuts. bought grid, gtr evolution. then a whole xbox, fm3 and the microsoft wheel. but i have a fanatec gt2 which i just got and FM4 would complement it well. i havent picked up the f1 games as there's a few f1 cars in gt5 and thats gotten me by. i play lots of other games as well other than racing so have to cut somewhere.