You can hook up the 8 ohm speakers, but will lose power. You can't afford to lose much as that amp is only rated at 15 watts at 1% THD. If you only want a really low volume, you will be OK. If you want more for a party, etc, then I suggest a bigger amp. You should not lose quality as much as volume with the 8 ohm speakers and you won't blow up the amp. You would get into more trouble running 4 ohm speakers on an 8 ohm amp which would run the amp hot.
For the best solution with that amp, though, you seem to know to run two 8 ohm speakers in parallel per channel to get to the 4 ohm load.