[I don't own an m4, so just guessing here]
You can measure the resistance on the two pins of the power supply where you connect the on/off cable - it should drop to 0 for a short period when it's trying to turn on the computer. Note that your multimeter might actually not be fast enough to detect that change.
The computer turns on when you short the On/Off pin. Try attaching wire leads to the mobo on/off pins and shorting them manually. See if it takes an abnormally long time to turn on after you short it (like 2000ms instead of regular 400ms). If that's the case, it might be either a defect of the mobo or some weird BIOS setting.