Hearing you all talk about the uselessness of grinders is truly puzzling. You need a grinder. A four piece one is better because of the kief benefit. And kief is certainly worth saving.
By grinding your weed, the benefits are easy to point out. It breaks the herb up, necessary for vaping and certainly beneficial for any other smoking. Breaking it up with a grinder instead of hands increases the amount of trichomes that you're able to touch with the heat, as well as preventing those lovely trichs from staying on your fingers. That THC that makes your fingers sticky after breaking up your weed?? Yeah, that could be going to your lungs/head instead. Breaking it up with your fingers is just wasting all that goodness. Meanwhile, putting it into a grinder to break it up also gives much more smoking area in your bowls, allowing for the flame (or vaporizor) to touch every part of the herb. If it's not properly broken up and the flame can't quite get all of the trichomes, you're not going to get as high.
As far as 4 piece versus two piece, the benefits of kief are multitude. Kief is at the bottom of your kief catcher is almost 100 THC--VERY strong. Sprinkle a little on your last bowl of the day, and you'll notice a much more intense high. Kief is also my preferred ingredient for medibles. You know exactly how much THC you're putting into your food, whereas it is tough to measure, very difficult to strain, and not as cannalicious when you use flower instead of kief. I have also heard countless people using kief in tinctures, to make hash, and BHO though I personally have not. My ex would even sprinkle kief on small bowls of tobacco when she ran out of herb. She claimed it got her high in times of need. Kief is certainly different than weed, and is just another tool to have to enjoy the plant.