Can I mix my current QS8 sides with QS4's? I'm thinking of purchasing a pair of 4's for the rear two channels?

Thank you

You should be fine with that setup. For 7.x rear speakers, there ought to be at least 5 feet behind your seating position, otherwise might not be much marginal utility in moving up from 5.x channels.

https://www.axiomaudio.com/blog/home-theater-speaker-placement

Others have said that there is some utility in using larger surround speakers with deeper bass extension, but I've been satisfied with surrounds that don't go below 80 Hz.

I have not heard the QS4 but think it would be fine to mix them (I own QS8’s and QS10’s.) … Axiom does a pretty good job color matching their line. Your receiver should calibrate out any loudness differences (as long as the QS4 is appropriate for the setup)

If you’re thinking of going with 4’s because of cost … maybe a refurb pair of 8’s might fit the bill … especially if you combine refurb pricing with a sale.