I'm a fan of the HF 4" grinders. Not that they're a quality tool or anything, but for far less than a quality grinder I can keep several on hand. Not having to change wheels in the middle of a project is a nice thing, and if something goes wrong just toss the thing.
I have a couple Makita 4" grinders. They're a decent grinder, but not worth the cost. At work I use Milwaukee. Way over priced, and triggers fail on a regular basis.
My 7" is a Makita. It's a killer! But, it's heavy so plan on getting a good workout if you're using it much.