A 4 piece herb grinder typically consists of four parts:
Top Lid: The top lid is the part of the grinder that you remove to access the grinding chamber. It usually has teeth or ridges on the underside to help grind the herb.
Grinding Chamber: The grinding chamber also known as the grinding plate is the part of the grinder where you place your herb to be ground. It usually has teeth or pegs that interlock with the teeth or ridges on the top lid to grind the herb as you twist the lid back and forth.
Pollen Screen: The pollen screen is located at the bottom of the grinding chamber and separates the ground herb from the fine pollen or kief that falls through. It usually has a fine mesh screen that allows the pollen to pass through but keeps the herb from falling through.
Pollen Chamber: The pollen chamber is located below the pollen screen and collects the fine pollen or kief that falls through. This chamber is typically smaller than the grinding chamber and can be accessed by unscrewing the bottom piece of the grinder. The collected kief or kief catcher can then be used for later consumption or pressed into hash.