I put a 100 watt Tube head through a cab with four 100watt speakers? (effectively making it 400 watts)

as opposed to the 120 watt cab that matches?

I would think nothing except you wouldn't really get the head to run at it's maximum potential.
it would be fine as long as its got the right impedance

Ibanez RG7321 w/ D-sonic in bridge

Peavey 5150 mk ii & b52 4x12 cab

line 6 podxt for recording

Quote by AsOneIStand
Head and Cab for $130? You don't need a head and cabinet, you need a psychological examination.
The head would run just fine. As long as you got the impedence right nothing would go wrong.
Quote by Powerhouse
I was in a nu-metal band when I was 15. I hated the top 3 strings (G,B, and E). I wondered why guitars even had them.