Code? Allow to use 2x4 bays for return?

Depends on local codes.

It's done all the time in my area.

2x4 won't flow that much air, maybe 200 cfm per bay.

2x6 is better for this.

Panned off floor joists can flow up to 300 to 500 cfm depending on the thickness of the joists.

Metal ducts are far less leaky but add to cost.

What about when the ends are 2x4 and the middle are built from 2x3 and there is a T in the wall also having 2x3 with a 2x4 at the end creating a screwy area with an electrical panel in the middle and wires in the bay. In short, no one bay is closed, it just keeps going because of 2x3 creating a 1" slot.

I don't quite understand the question.

Reducing to 2x3 reduces airflow too much.

Having too many wires in the cavity can do the same.

The ends of the cavity have to be closed or air gets drawn from the wrong places.