The single stroke four is three quick notes leading into a fourth, accented note. The three lead-in notes are played as a triplet, so the pattern has a snap to it: da-da-da-DAH.
Sticking
R L R L
Capital letters are main notes. Small letters are grace notes played just before the main note.
How to practice it
Play it at a slow tempo with the click, counting out loud.
Listen for evenness: every double should have two equal notes.
Raise the tempo 5 BPM at a time. When it gets messy, drop back 10.
Think of the first three notes as a pickup into the accent. Keep them soft and low so the fourth note jumps out.
Where you will use it
It is a common short fill figure and shows up everywhere on the toms, especially leading into a crash.