I started playing bass 3 months ago and so far got pretty far in playing with 4 fingers on the right hand(right handed)

are there any different methods that would improve finger strength and speed?

Pick up any classical guitar method. The Carcassi method is particularly good. I regularly use those exercises on bass.

Constant metronome practice at slower speeds will build most of the stamina you need.

Also, here's a thread I created on a four-finger plucking method I use if you're interested
http://www.talkbass.com/forum/showthread.php?t=225211

You may want to check out Richard Bornman.
http://www.soundclick.com/bands/pageartist.cfm?bandID=13286

He plays guitar but he uses fingers and alternates between thumb-index-thumb-middle-thumb-ring.

He has an amazing command of the instrument. I have been playing bass this way for a while though I'm just starting to incorporate the ring finger. I primarily play thumb-index-thumb-middle.

Matt Garrison notwithstanding, I think Richard's technique is the most efficient.

ok kool. thanks.