I have several 4-ft x 4"x4" pieces of wood, and I need to shave off a few millimeters of wood.
Can I use an angle grinder for this? It doesn't have to be perfect, and I would only be shaving the millimeters from a 8-inch section of the 4-ft post.
You can use it, they work for coping and other sundry shaping. Then again, the fact that you can make a statue with a chainsaw doesn't mean that every carpenter ought to have on in their toolbox. A plane will give you a much flatter surface and probably be just as quick. I would suggest avoiding the grinder unless you can accept a wavy surface, since it will be difficult to keep the wheel flat and wood abrades much more quickly than metal or stone.
Speaking of chainsaws, I would suggest avoiding the chain-wrapped wheels for grinders in their entirety. I've never heard anyone suggest that they're a good idea. If you haven't seen them it's a grinder wheel with a chainsaw blade around the perimeter for sculpting wood.
A better solution is one of those abrasive flap wheels which mount sandpaper flaps to a grinder disk. It's not a replacement for belt sander but it's an economical solution if you already have a grinder.