with 7.1 speakers you dont have 4 rears....
You have 2 surrounds (preferably diapole) directly to your left & right with the other two used behind you.
with 5.1 the surrounds are supposed to be at 90 degrees to the listening position...everybody get this wrong.
p.s a pair of surround rears v a single rear surround makes little to no difference in most average lounge rooms....the surround back channel for both rear surrounds speakers is mono.( a mix of both normal surround channels).