I wanted the wing area and wetted area to be double the original. So with the same shape all dimensions have to be enlarged by factor of square root of 2.
Drag area doubled, parasitic drag doubled, but with twice the power, roughly the same speed.
Twin engined Mossie combined the duties of navigator/bombardier. Why would a 4 engined Mossie be any different?
When in war, I would like to have at least one gun than no guns at all. And tail gun was/is the most important of them. AFAIK B-52's still have them.