check that the bike was never stunted, that it was maintained well, and has no evidence of having been crashed or otherwise damaged.
if it doesnt show evidence of the above, you're good. the F4 isnt as popular for stunters as the F4i is, but its still an INCREDIBLE machine to ride.
I liked the F2 myself, its my idea of a perfect sportbike, integrated signals up front, easy to do on the rear, very reliable and well constructed.