This is just me on my recto (i'm assuming it's the same as a RK), but I'm setup like either:
2 4x12's
(cable 1) 8 ohm jack on cab to 4 ohm jack on head
(cable 2) 8 ohm jack on cab to 4 ohm jack on head
or
1 4x12
8 ohm jack on cab to 8 ohm jack on head
There are other ways to do it (cabs in series, etc) shown in the manual.