The MMATS has a max input sensitivity of 2.5v and my deck has sub out of 4v. Is this going to be an issue? Will the amp be able to accept the higher voltage?

 

It will not be an issue.

 

It will definately not be an issue. You will likely only get more than 2.5 volts from the preouts at close to max volume. Even with all the hype around high voltage out there, most amps never see high volts, and if they do, your ears are bleeding or you are outside the car with a DB meter inside