Nemo,

Yes, technially a four channel will work. I have seen people use the left stick as a ballast tank fill-drain. However to make things easier you either have a switched on-off-on switch or some sort of proportional slider to control the ballast tank(s). I have never seen a four channel radio with one. If there is one I stand corrected. A six channel typically has the capability to do all those functions.

best regards,
Pete