I'll just give you a brief explanation on it. The purpose of the gain control is to match HU's output voltage with the amp's input sensitivity. So if your HU says it's 4v, then set the gain to 4v or close to it.
If that is the HU's output voltage then yes. You turn the gain to somewhere around 6v. Doesn't have to be exact. You can operate the amp even at 5v without much problem. On my amps, I have it 1v less than the HU's.