this is just an off-the-top-of-my-head response - but i would not single out those fingers by themselves. i'd pair them with 1, 2, or 3. exercises in thirds always helped me, and also doesn't wear out those weakling fingers.
some people take it from the other perspective and start working trills. i just never could do that. my fingers are connected by some really strong ligaments or whatever and i couldn't trill a 4,5 if i tried for 100 years. i mean, i can do a slow one - but i have a very short pinky. it's a little above my fourth finger joint (tip of pinky). this makes trilling a sort of really curved finger affair. which i could do if i tried - but i don't want to.
just scale practice alone may not strengthen them as much as the 'thirds' exercises. my older college prof gave us five-finger exercises (about 10 or 12)
i'll try to remember:
123454321...(in all major/minor/dim7th keys)
121234343545432321...
(you can do these hands apart and then together)
1324354231...
31, 42, 53, 42, 31...
things like this. over and over and different variations (say two times 31, 42, 31, 42...then 53, 42, 53, 42)
it helps you quickly go through all the keys with the right sharps and flats, too (just for the five fingers).
scales are another good exercise - because they do strengthen your fingers after awhile. my teacher used to have us 'block' scale patterns to the thumb (play all the notes together)
CDE FGAB CDE FGAB - you play them really solidly up and down the keyboard. having the other fingers to support your weak ones - means you learn how to 'compensate' and still get a good sound from the 'whole.' no notes should be weaker sounding than others.