#2
Amplifier output  (8 ohm min)> proper speaker cord > 8 ohms (right side) cabinet input.  This will work fine and use 1/2 the cab.  Attempting to run the whole cab at 4 ohm will probably fry the amp.  Enjoy!
"Your sound is in your hands as much as anything. It's the way you pick, and the way you hold the guitar, more than it is the amp or the guitar you use." -- Stevie Ray Vaughan

"Anybody can play. The note is only 20 percent. The attitude of the motherfucker who plays it is 80 percent." -- Miles Davis

Guthrie on tone: https://www.youtube.com/watch?v=zmohdG9lLqY
#4
What about it?
"Your sound is in your hands as much as anything. It's the way you pick, and the way you hold the guitar, more than it is the amp or the guitar you use." -- Stevie Ray Vaughan

"Anybody can play. The note is only 20 percent. The attitude of the motherfucker who plays it is 80 percent." -- Miles Davis

Guthrie on tone: https://www.youtube.com/watch?v=zmohdG9lLqY
#5
So generally speaking the rule of thumb with impedance is match it whenever you can, which unfortunately isn't straightforward to do with this setup.

On this amp as you can see it says "minimum 8ohm", meaning plugging straight into this cab mono will likely cause issues and possibly damage to your amp.

The good news though, is that if the cab is 8ohm a side, it likely has 4x16 ohm speakers and therefore can be easily rewired for 16ohms mono, which will be okay for your amp. You can even get a new jack plate if you like that would allow you to run 8ohm a side stereo or 4 or 16 ohm mono (Google plug n play jack plate).

If you dont have the skills to rewire the cab yourself, or don't want to pay a tech to do it, then as Cajundaddy said, you can just use half the cab by plugging only into the right hand side jack. But honestly, why would you want to lug around a 412 if you can't even use all the speakers? 
'66 Fender Bassman, '82 Marshall JCM800 4210, '86 Laney AOR 100, '90 Marshall JCM900 MKIII 2500, '97 Marshall JCM600, Silverface Mesa Dual Rectifier, Peavey 6505, Marshall JCM2000 DSL-100, VOX AC30CCH, Marshall JMP-1 + EL34 50/50