The fourth form is a bit unusual. Where each of the other forms is used in pairs, two ones, twp twos or two threes, the fourth form pairs with either the third form or the first form.
The reach is a little challenging, the first finger plays on the second fret, the second on the fourth fret, and the third plays on the sixth fret. It’s pretty much like the first form except that third finger stretches to the sixth fret.
Here’s the form exercise on the D and A strings.
There aren’t very many tunes that use just the fourth and first forms. Monaghan Twig (sometimes known as the Tinker’s Apron) is one of the few that I’ve found.