A 4 piece grinder is a bit more advanced. Sometimes they come in 3 pieces too but the main advantage is that you have a kief catcher. This is the pollen of your herbs that fall down into the bottom compartment. Eventually, it fills ups and can be used on top of your dry herbs.
There is often a lower bottom compartment which is used for holding your herbs as storage.
How to Clean Them?
Cleaning metal or aluminum are the easiest. Solid herbs tend to not stick to aluminum as well as other materials. using a simple brush can get the job done. If there is left over gunk, try soaking it in warm water, salt and some isopropyl alcohol.