What makes you think you have four tanks ?
You looked at them or do you have four dump valves ?
Post a pic, because I've never heard of that and would like to see botht he tanks and the utility closet full of dump valves
Now our large redwood had 3 tanks, galley, grey, and black... but since no one knew what went where
I usually just tied the galley and grey and black together with a closed valve on the sewer outlet and opened all three tank valves...