manual : [url="http://www.marshallamps.com/product.asp?productCode=4100&pageType=DOWNLOADS"]http://www.marshalla...eType=DOWNLOADS[/url]
It's 8 or 16 Ohm only:
[i]Impedance Selector Switch: Two-way switch for matching the amplifier to speaker impedance. Amp heads feature 8 or 16 Ohm operation. Combo versions feature 4 or 8 Ohm options.[/i]
Guessing they used the same backplate on the combos as the heads.
So nope you can't use a 4Ohm cab with this one.
If you wanted to use 2 cabs they would have to be 16Ohms each, and you would set the switch to 8Ohms.