Correct. 4 valves. 4 tanks. 2 dump valves.
For mine, the FRONT valves are left and right. The right valve towards the garage is the front black. The left valve towards the TV is for the front bathroom sink and shower (grey)
The rear valves are up and down. The top valve is the kitchen sink (grey) and the bottom valve is for the rear bathroom toilet only.
It's confusing, but the front two valves and the top rear valve all dump through the front dump drain. That's Two grey tanks and one black tank. If you never use the rear toilet, then you will never use the rear dump drain.
I always pull the back front valve first (black) and then flush with both grey tanks. All through the front drain dump.
good lord, I'm even confused now....