I have FS4 and 5.1 and I've got to admit I like 4.0 better. Have you noticed that every time FS gets an upgrade you have to overhaul you machine to run it? I've got a 486 DX4/100 with 20 megs. It is really my wife's graphics station, but she lets me use a mere 5 megs and a little desktop and floor space for my controls.
I have 200 square miles around my house here in Batesville, MS. programmed in from FAA sectional charts. I've also downloaded gobs of planes FREE OF CHARGE.
Don't give up on old 4.0 yet. Oh yeah, anyone with a copy of Aircraft and Adventure Factory for FS4 that wants to get rid of it...I'll gladly take it off your hands.