Yes, the 4th finger is smaller, and?

It is sometimes annoying that the 4th finger is smaller than the others (though ours is likely twice the size of those fabulous 5 year old prodigies with great sounds and technique on Youtube) Still, we need to make equally beautiful sound, regardless of the finger. Try this: pick a note - any note. Now, play it with vibrato, using each finger. Can you make the 4th finger vibrato sound like the 1st, 2nd, or 3rd finger vibratos? If not, start by practicing with intent and awareness of the way the hand needs to rock for each finger, so all four could - if you wanted and it was appropriate - sound exactly the same in a blindfold test.