Can I just use thhn electrical wire for an amp?

Yes, that will be fine. Only downside is that it won't be as flexible.

Never any down sides about going with a bigger gauge wire. If anything, it has the potential for better performance. A well fed amp, is a good amp. I don't know if the 4 gauge you're going with is an upgrade from a smaller gauge, but if so, remember to give it the right fuse. The kind of gauge you use usually determines what size fuse you need. Good luck. 

Good job completely missing the question^^^^

Joel114 said:

I work as an electrician and have enough 4 awg copper thhn stranded electrical wire to run from my battery to the amp I am going to install. Could I just use that instead of buying some EB Flex? Is there any downsides?

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We can safely assume you know how to use ampacity chart and #4 is adequate for your amp(s).  The main difference in "car audio" power cable is flexibility and sometimes better/thicker insulation.   Just be mindful about putting wire anyplace where it might flex or rub on something, otherwise, copper is copper and electrons really don't know the difference.

If you live in a cold climate, you will want the more flexible wiring. Getting thhn to flex when its cold is like trying to bend the branch of an oak tree. 

Yes It'll work just fine

I've used zero gauge thhn in several cars and I've never had a problem. Just have to be a little more creative with the routing.

With car audio where there is DC power instead of AC compared to the house,  the current tends to loose over distances from the car battery to the back of the trunk area.  Simple way to know your power requirements is to add up your amplifiers fuse rating and add 10% ( due to loss) and then go shopping for the appropriate wiring kit.  In most every day driving 4 gauge or 2 gauge is sufficient unless you plan to enter the SPL lanes then back to the drawing board for you!