John Patitucci's "Spider" is a great exercise for the four fingers:
I've recommended this exercise before, so here are some personal remarks I've posted about it as well:
-This is to be played using strictly one finger per fret and the pattern is 1-3, 2-4, 3-1, 4-2.
-60 bpm is a good tempo to start playing it.
-Don't move to the next repetition until you feel comfortable with the current one.
-(This is a very personal one) Keep the notes ringing the longest you can (specially on the slower sections - quarter notes). No matter if the notes overlap. Playing the exercise this way forces you to keep your left hand fingers in place the longest possible time and that's a demanding requirement. You'll have to adjust your right hand technique to accomplish this (plucking the strings with an upward motion, without resting your finger on the adjacent string).
-If you understand the fingering pattern, you can extend the exercise to an instrument with more than four strings (although I can't imagine playing the last repetition on a 9-string bass).
Hope this helps.