.... and it's either going to be really cool or break everything.

I have a Fender M-80 head and it has 2 inputs.

Soooo... What would happen if I plug my guitar into one input like usual, but then sent the output from the 4 ohm through the other input and then sent the 8 ohm output through a cabinet.

Would that do anything or just be a total bust?
It won't work, the input of a guitar amp is built to tolerate a low-level signal that comes from a guitar. You could damage your head.
no it won't work. even if it did, what the hell would you do that for anyway?
Gear :

Gibson Les Paul Special
Fender Koa Stratocaster
Korg Pitchblack
Dunlop 535Q Crybaby Wah
BBE Boosta Grande
Fulltone OCD v4
EHX Russian Big Muff Pi
Vox Night Train
Egnater Rebel 112x Cab
Marshall Class 5
No. Inputs take in a guitar signal. No signal is produced from an input, just to an input.

How do you say "I'm okay" to an answering machine?

G&L Tribute S-500 -> Ernie Ball VP -> Boss SD-1 -> Fulltone OCD -> Small Stone Phaser -> Danelectro Cool Cat Tremolo -> Boss DD-3 -> Boss RE-20 -> TC Electronic Hall of Reverb -> Boss RC-2 -------> Modded Crate v18 Amp