The simplest solution (ideally) would be to trigger a relay that in turn will close the circuit to the ACC wire on the M4-ATX.
You can't use both the P2140 and M4-ATX at the same time....hmmmm....I guess you can if you really wanted to, but the only way I see you using the P2140 right now (without blowing it up) is in the same way you would use a relay. Between the P2140 and a relay in your case I'd pick a relay.
Here's a couple of threads on it already:
-Do you have a car alarm tied to your key fob?
-Do you have a key fob or you manually lock/unlock your car?
-When do you want your computer to turn off?
-What happens when you unlock your car? (Dome light comes on, turn signals flash, your car sings, etc...)