400mm f/5.6L old technology?

About 3 years ago i got myself the 400mm f/5.6L lens for my 20D camera, mostly for photographing birds and other wildlife. After seeing many sharper photos of wildlife and birds taken with a 500mm f/4L Is (first introduced 1999) I have come to the conclusion that it is a much superior lens. OK, I know all about the advantages of a larger lens opening and IS, but could the older technology of the 400mm lens (first introduced in 1993) be a factor in its performance?

I still love my 400mm lens for its sharpness, weight factor and of course initial price, but . . . I often envy people who have the 500mm lens.

Any thoughts?