If you never sell anything smaller than a 4pack then I guess it makes sense that this is your “base” product. But if you set up your case product as 6 of those, then you won’t be able to return less than an entire case? Am I right? So you shouldn’t be able to return a single 4pack.
There’s a feature request for this:
Or are you managing the return in some other way?
But as far as the credit note goes, if you had a price for the 4pack on your wholesale pricebook, then that should “just work”. Shouldn’t it?