Will this work properly if when wiring a sub-panel for 240v can one of the 3 aluminum 4 gauge wires in the pvc underground pipe be used as the neutral wire if that wire is bare?

I intend to attach the ground bus at the sub-panel to a ground rod using 6 gauge bare copper wire.

Yes a bare wire can be used as a neutral when it is part of a multiwire cable.

However, a 3 wire feeder can only be used to feed a subpanel if the panel is in a separate building, and there is no other metallic path between buildings, such as telephone lines.

If the 3 wire is used, the neutral and equipment ground bars must be bonded together, usually with a green screw into the neutral bar, and at least one ground rod must be driven and connected to the neutral bar.

Must bond any water lines to the neutral. and any other building metal systems, except natural gas.

If the subpanel is in the same building as the main, must use a 4 wire feeder.