PA system is totally different from home audio system. Thier not design for pleasure so volume is preset to comfortable level. And yes you can put 4 speakers and up as long you don't go over you actual power output.
example: If each speaker is producing 15 to 20 watt then you can put 5 speakers. Also, speaker use on the system uses 100 uF or is it 150? sorry, can't remember.