Nah it's not going to hurt anything. You can do that, it won't be a problem. However, your 8 ohm speakers might sound a lot quieter than the front due to them being higher impedence. I'm just guessing though, without knowing all the information. There are so many variables

Anyway yeah, that shouldn't hurt anything. Car speakers are 4 ohms usually, home speakers 8. Subwoofers are the exception, they vary widely. A lot of people run 2 ohms or even less to eek more wattage out of their amplifiers.

What you probably DON'T want to do is hook up some 4 ohm car speakers to your home stereo Lots of home stereos are cheaply made and won't survive operating at a lower impedence than they were designed for. The amplifier circuitry gets hot and either shuts down (if it has a protection circuit) or just burns up.