Most heads, you can select the ohmage, ie, 4, 8 or 16 ohm.
It's the cab you usually have to worry about!
My cab is 8 ohm, therefore i have an 8 ohm hotplate
It doesn't matter whether it's a 2x12 or 4x12! It's only starts to get complicated when you connect 2 cabs instead of one, regardless of how many speakers in them
So, if your cab is 8 ohm, set your head also to 8 ohm and buy an 8 ohm hotplate :D
Simple!