Why does my new alternator have 4 posts, not three?

true that,
the 2.2/2.5 eng family ("k" platform) use rubber bushing inserts in the alt mounting bracket.

as stated though, there is no such thing as "ground overkill". I would
add the extra ground.

Oh, I will... I will have a few ground wires running around that engine bay.

That's one thing I learned working with car audio: You can never have too much ground. Usually when you buy a amplifier wiring kit, the ground wire they provide is insufficient... I kind of think it is that way when it comes to cars in general... The local parts store said it was fine to just run a ground to the radiator support and call it good.....