How can I square the end of a 4x4 post by grinding or sanding?
Do you have a saw? Any sort of saw, be it hand saw, circular saw, hand or power miter box. These are all appropriate tools for squaring up an end like this. Even a jig-saw with a long enough blade should get you a cut that's close enough to finish sand.
An angle grinder would not be a good tool for this, which is probably why you're not finding any wood cutting blades. Usually an angle grinder is not a woodworking tool. You can use it, but you'll end up burning the wood with a grinding disk, and probably with a standard cutting disk, too.
You can use a hand plane, though planing end-grain is very difficult and requires a very sharp blade and proper setup of the plane (check out the Woodworking sister site for plane setup details - I know there have been questions about that there). If you don't already have one, you're going to spend more money, time and frustration trying to get this setup than it's really worth, especially if you expect this to be a one-off situation.
You could also just use your sander with something like 36 grit paper. You'd be surprised how quickly that will chew away wood. Get close to your finish line with the 36 grit, then switch to something like 80 or 100 grit to get really close, then switch to a higher grit if you want/need a finer finish, or just finish it up with the 80 or 100.