Are there any excercises out there specifically meant to train you fourth fingers?
I know that there are pieces for the left hand alone, but I wasn't sure about this.
Be very carefull! We all know story about R. Schumann's 4th finger and where he ended his life

There are all kind of theories about developing its independance and strenght. I am for one who believes that the most important thing is to have not physical, but menthal control over it--4th always works in conjunction with 3rd and 5th, and its the best never try to go against of nature.
I would start with slow 3rd-4th trills in triplets, with accents on every first bit of the triplet. Then modify it like C-c#-c-C#-c-c#-C-d-c-D-c-d-C-eb-c-Eb-etc. and come back to D, and then next transpose the same from C#. I put capitals on accents. Do exactly the same with 4th-5th.
Then (probably somebody would hate me for that) Chopin 2nd etude very slowly, without chords, only upper chromatic melody, with the fingering indicated. If you feel uncompfortable with that just play chromatic scale with etude's fingering. Do exactly the same with your LH. In all these excersises pay extremely careful attention to eveness of sound, smoothness of rhythm, and feeling of absolute comfort in your hands.