4 gauge power wire with 8 gauge ground....going to hurt anything?

Just curious, I'm running a very moderate amp, it's like 100x2 and only powering some door speakers. I ran out of 4 gauge wire, but do have some 8 gauge lying around.

Is this going to hurt anything? I know ideally you should use the same gauge ground as you do power, but for such a low power amp, is this going to hurt anything?

8 gauge will be fine

There are 2 things to consider when running your power cables. One is how much current will it be carrying and two, how long a run it will be. For a run from the battery to the back of your car 4awg is plenty big enough in most cases. Unless you are running a ground cable all the way back to the battery there is no reason to use 4awg cable for the ground run. Most grounds are only a few feets from where the amps are mounted. Since the run is so much shorter 8awg cable is fine.

while the power wire runs the length of the vehicle, the ground is only about 2 ft long. Thanks, I don't have to worry about burning my car down anymore.

Now I just need to get my subthump amp rack so I can tidy this all up, it looks like hell right now

There aren't many 100x2 amps you can stick 4 gauge into :D Anyway, short 8 gauge will work fine with long 4 gauge using much more hungry amps.