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.