I have a problem recycling to the 4th finger. I can repeat the 3-2-1, but I can never get my finger back quickly enough to make it sound smooth. What am I doing wrong?
It's due to the anatomy of the hand. Place your fingertips firmly on a tabletop, hand in a dome shape, as though in five-finger position. With steady downward pressure on the fingertips, lift only the thumb. It should easily fly up, independent of the rest of the fingers. Return the hand to the original position. Now lift only the index finger, this is easy. Do the same to the middle finger, it also can move upward independently. Now, try to lift only the fourth finger, somehow it is paralyzed! Moving on to the fifth finger, it easily pops up on its own.
It is extremely difficult (nearly impossible) to lift the fourth finger over the third finger. Keep that in mind when practicing coordination.
You may also be breaking the legato when going from 1 to 4. Keep it connected, without applying excess force; and don't lift the fourth finger in an attempt to reach the next note; just gently move the whole hand into position, using the arm as a lever.
For repeated notes, I prefer the fingerings 3-1-2-1 or 2-1-2-1 or 3-1-3-1. To me, a repeated note is simply a trill played on the same key. The common motion is forearm rotation. Using the thumb every other note, facilitates this fluid rotation. Of course, use the fingering that's best for you; it may be different than what the score indicates.