Hi I'm selling a Marshall valvestate 4x12 cab and somebody asked me if it would work with their MG head and if it will they'll buy it. I'm 99 percent sure it will work fine but I just wanted to clarify as I've only ever used a valvestate head and a Crate head with it

Yes it will work. Any cab will work with a Solid State head.

Gibson 2005 Les Paul Standard
Fender Road Worn Strat w/ Noiseless pickups
Marshall JCM 2000 401C
Marshall Vintage Modern 2266
Marshall 1960A cab (Dave Hill from Slade's old cab)
Ibanez TS9DX
EHX Little Big Muff
Freshman Acoustic
Yup. In case of solid states there is only minimum ohms you need to worry, which is usually 4 ohms (it should read behind the head) and that is the load where the amp is capable of putting out the watts its advertised at. If the load is below that, the amp overloads and goes kaboom. Higher ohms only reduces output and higher you go more watts are wasted and that might even be desireable. I'm running a 16ohm cab with my solid states myself.

ESP LTD F-50 + Tonezone
Cort EVL-Z4 + X2N
Cort EVL-K47B

Marshall Valvestate 8100
Randall RG1503
Bugera 333
Peavey Rockmaster preamp

Line6 Pod X3