First of all, that 4-pin power connector is NOT an ATX power connector.
What you are doing is jury-rigging an ATX power supply to power your setup.
This will be the same for an Opus unit, an M-series or a DSATX, as they are all ATX power supplies with the 20-pin (or 24-pin) ATX power connector.
You are better suited to get a regulator that provides the regulated 12vDC you need, like a Carnetix P2140. It also has a startup/shutdown controller built-in.