If the first return is executed, then the method is all done. No other code in the method is executed (including, of course, that second return.)
This code is quite odd -- there are all kinds of things that I'm sure you didn't intend here.
You should probably ask for
more help understanding what's going on.
One important thing I'll get you started on: after this code, the array m4a will contain 20 Mix4a objects. Every one of them will have a "counter" member equal to 1. Can you explain why?