The two-piece grinder is the most popular, but it doesn’t have a separate storage area connected for your grounded herbs. When you grind your favorite herbs with a two-piece grinder, the herb will be in the grinder itself.
A four-piece herb grinder has a separate container attached to the grinder that catches grounded herb and a kief-catch container. This is a great feature, but slightly more complicated than your average two-piece grinder.