still as delboy999 said, if you wanna play loud get an amp, if you overload the headunit, you can cook the head unit and the speakers - when under too bigga load the headunit will clip, which will fry the speaker's voicecoils. Keeping that in mind, it also depends on your speaker's sensitivity..i have a set of super-sensitive pioneer 7x10" square cone 4 ways running off the eb headunit lol - they work fine, but i know the headunit's limits so i dont turn it up too loud. there are also other factors, there could be something shorting out somewhere when all 4 are connected, like on channel interferring with the other one. My mat currently has the same problem, but hes too lazy to fix it!
Hope this helps - Blake.