As a result of hours plowing through exercises in the Belwin exercise books, followed by the Schirmer fingering exercise book, followed by Czerny exercises, I have extensive control over fingers 4 and 5. This was the goal of my mother because before piano, I was always helping finger 3 with finger 2 as a result of cutting the end of finger 3 off at age 3. As a mechanic, I can thread a nut on a threaded bolt, with fingers 4 and 5 opposing, unassisted by 1,2,3, blind, around a corner, while I am upside down with my arm stretched as far as I can reach. I don't really believe the preceding statement about 4 and 5 not being independently controlled, as a result. It is also true, that many pieces sound better with the top notes louder, so that while you could play louder with 1, 2, or 3, it is musically better to play louder with 4 or 5. For example, listen to the 3rd movement of Beethoven's Moonlight Sonata, it has a lot of 4-5 runs and trills. You should develop this skill over a period of years, not weeks, to reduce chance of injury. Follow the guidance of an experienced and trained teacher if you want to achieve this skill. The skill with 4 and 5 would also help if you want to be a dentist or surgeon.