I think I have the same number of 4-strings as 6-strings, and quite a lot more 5-strings. Looking at the Bass Gallery, there's about five times as many 4s on sale as 5s, and at Bass Direct it's about two to one. Those, of course, are higher-end instrument shops - GAK have 455 4-string models and 121 5-string models, so though 5-strings will probably never overtake 4-strings, they're not exactly niche things. Both Ibanez and Cort when bringing out their new headless models seem to have concentrated more on 5-strings than 4.
I don't use the 4-strings for gigging because I've learnt everything on 5s and although I could revise my fingering, I wouldn't play as smoothly and there's absolutely no point. I do take them out to open mics now and then, just to let them know I still love them.