I'm trying out for a band and right now I have to take buses to get places cause I don't have a car. My amp is heavy as hell(combo luckily not half stack) and I wanted to know if the head can be ran through a PA speaker (cause they have one I can use) cause my amp has a 4 ohm output for a half stack.
It's not going to sound ideal because pa speakers are typically full range and a guitar amp's speaker is wound/voiced differently to suit the range of the guitar. Which amp do you have though? The added speaker won't increase volume appreciably - your combo is either going to cut it or not.

You'd also have to check the ohm rating of the pa.
Ya I know its made for voices but just wanted to make sure it doesn't ruin the speaker. I have a spider 2 head, its 120 watt(60x2) so I'm geussing volume should be fine.