As of now there is virtually no difference between 4x agp and 2x agp. The only difference being that 4x agp can become unstable at very high fsb settings(+140mhz on Intel systems, don't know about AMD though). I always keep my agp setting at 2x on my Asus CUSL2, which, supports 4x agp.
4X agp is of no use at the current time, because there are no performance benefits on current games, and because it can tend to be unstable.