I found out somehow that my Crate 120 watt stack is only rated 120 watts at 4 Ohms. And I also read somewhere that more Ohms require higher output. I didn't know that until lately. So I started wondering, "Is my amp really 120 watts?". Technically yes, but not all the way. If I buy an 8 Ohm cab, would it work, but at half the wattage? Not that I am, just a question of mine. And is it possible to plug in a 16 Ohm 100 watt head into 4 Ohms, wouldn't it essentially quadruple the power into 400 watts? I didn't know where else to put this, so if it needs to be moved, please move it mods.

One main reason: The Rev Jr. by Krank has i think 4 and 8 Ohm output, which you select manually. So would it put out more than 15 watts to a 4 Ohm cab if it's coming out the 8 Ohm output?
I got some good guitars, yo.
The wattage doesn't double when the impedance is halved. It changes, but not by that much. Also, running a 100 watt head rated for 16 ohms into a 4 ohm cab will blow up the head. Ohm ratings are there for a reason, you can't just go messing around with them.

Usually you are safe using a cab twice the impedance of the head. The changes there are mostly tone changes, the volume doesn't drop a whole lot.