I am adding a 4 circuit subpanel right next to my main panel. The subpanel will have 2 20amp circuits and 2 15amp circuits. What size cable do I have to run from the main to the sub?
Depends on what you are feeding with the 20 and 15 amp circuits.
Myself, I would put in a 40 amp breaker with a #8 4 wire cable. It's called 3 wire with ground.
That's 2 hot 1 neutral and 1 ground.