4 channel sets are usually two of two axis sticks. Extra functions might need switches rather than awkward stick waggling, so a higher channel count (5,6,7) might make more sense even if not all are used.
Since planes are overwhelmingly the main market for more comprehensive radios, that is what the market mostly consists of. Ailerons and elevators and landing gear and suchlike need translating into something useful.
Otherwise, what Dave said.