Provided you dedicate a small portion of your life (all us petrolheads know that isn't really true) to motoring, cars, racing...appreciating all the intoxicating thrill and beauty cars/racing bring into our lives... you're going to LOVE FM4 to death. And you need not go online in order to appreciate what T10 has to offer with iteration no. 4.
The car count is insane, the track variety is good. And even though you get just 2 lap races in Arcade mode (1 lap on the really long ones like The Ring and Kaido), you can use them as platforms to learn the AI and perhaps experiment with different cars while taking photos. Free Play/hotlappin' allows you to do as many laps as you want and IMO is an excellent way of understanding specific cars/tracks, experimenting with countless tunes or just getting a leg up on your lap times.
AI is pretty decent in Arcade more, but there are times in the Events List, where it shines and is very challenging. World Tour AI can be a joke, just so you know!
Getting an X360 just for FM4 you say?....Absolutely! You're going to get sucked in by the depth and physics, the hair-raising engine and exhaust notes, not to mention the endless tuning and livery editing options. Despite absensce of weather effects and night driving, the graphics are some of the best you'll see in a console sim.
Cheers maing! I were you, I'd spend zero time pondering over this - you're missing out on one of the BEST driving sims in console history "FORZA MOTORSPORT 4".... !