I would have a 4-motored plane use two channels for each pair, rather than all four running from one, with the inboards and outboards separate.
Mixing to get both work with the throttle, but have the "not throttle channel" motors on a variable channel.
I've done this with twins using Futaba equipment, both mixed to channel, with the odd one on channel 6, which has a knob which permits controlling that motor separately.
This for exploring single motor flight.
I found that more than 40% power on the running motor would be too much for the rudder authority to let the plane be turned towards the running motor.