What kind of fuse are you using for your 4 channel amp. If its the glass type "agu" replace the fuse. These units are known to fail if its a cheap unit. I seen this to many times with this type of fuse holder "cheap units". The fuse will look good but is really blown. Or the fuse is bad and will put 4 volts because of a bad connection inside of its self. High resistance. Next check the ground to the amp. Give me a call or a text if you need futher help. Anytime is fine. Des