4 fingers is a lil crazy cause of the shortness of the pinky you hand has to take up a weird position to have all your fingers arced to the same length
3 fingers is a little more practical imo many bassist have done it billy sheehan, john myung, steve bailey and i think i remember seeing john entwhistle use 3 too(for the my generation bass solo when he doesn't use a pick).
some of them adopt different methods to using 3 fingers for example:
1 - Index finger
2 - Middle finger
3 - Ring finger
Billy Sheehan's method (the one i use)
Finger order to get 4 note for chromatic scale
3 2 1 3 2 1 3 2 1 3 2 1
F F# G G#.....
notice that no finger plays 2 notes its alternates in order from ring to index
(correct me if i'm wrong..i think john myung's method is similar to sheehan's but instead its index to ring)
Steve Bailey's method
Finger order to get 4 note for chromatic scale
1 2 3 2 1 2 3 2 1 2 3 2
this method is a little tough basically you go from index to ring and back to index
all in all most bassist think 2 fingers is more than enough (look at how jaco he's wicked fast but only uses 2 fingers
) but if you really wanna use 3 go ahead and get billy sheehan's and john myung's books and dvd they tell a bit about the technique
hope this was of help