Darren is pretty much right on the money.
The best thing to do to build finger strength (hand strength in general) is just to
play. Concentrate on using all of your fingers.
Use the "4 frets/4 fingers" rule. If you are spanning 4 frets in anyway (scalar, chordal, etc) make sure you aren't
pushing your pinky out of the way to use your ring finger on the same fret.
When I tought for a living, I caught a number of players doing that often. Many players, professional and otherwise, still do it. Drives me crazy.
I had a slightly weirder problem; I had a weak ring finger. I never used the thing.
I do have fairly even hand strength now across all four as I've made a concerted effort and done some very helpful exercises I'd developed for finger strength.
Try this one:
1) Put the pick down (if you play with one).
2) Put all four fingers on the G string on frets 5 6 7 and 8 (C, C#, D, D#; chromatically).
3) While keeping your first THREE fingers on the G STRING firmly fretted, lift your pinky and hammer-on the 8th fret on the D STRING. Speed isn't necessarily important, but you'll find as you get stronger, you'll do it faster anyway.
4) Do each one for a minute or so, then move to the next finger. But always keep the fingers that are not hammering the D string firmly planted on the G string below it.
That exercise has been great for me alone. It helped me recoup from a pretty nasty hand injury about 8 years ago.
I have a ton of exercises, one day I'll do some sort of thing and post them all. They all help with right/left hand syncopation and finger independance. Most are pick-based, but you could certainly do them fingerstyle if so inclined.