Watch the Left-Hand Quads online guitar lesson by Ben Lacy from Two-Hand Groove Guitar
To play a quad we just add one hit on the end of a triplet. The simplest way is to come back onto the middle finger, so: ring-middle-index-middle. Try it with me.
It may not seem like there's much going on here but these make great fills. Have them ready for when you are really in a groove.