A 4ch amplier is essentiall two 2 channel amps in a single housing. Each pair of channels has it own indivuals gain settins.
Additionally amplifers which are capable of being Strapped together would use a special cable in order to automatically gain match multiple amps to one another.
Kicker subwoofer amplifers for example have this abilty. one amp becomes the master and the other amp is the slave.
This allows you to take 2 amps and put out double the power at double the oms.
example. The kicker kx400.1 is 400watts rms @ 2ohm. If you strap two together you get 800watts rms @ 4ohm. That quad 4ohm sub could then be wired to two kicker amps. 2 voice coils per amp.
