OK, so this is odd, it has one switch for the impedance setting, so for 100w it's either 8ohm, or 4 ohm, and for 30w it's either 4ohm or 2ohm
Now too 100w mode like that is usable, and much like mesa, ie, either one 8ohm cab for 8ohms, or two 8ohm cabs for 4ohms.
The 30w mode, however, has to be one of the most ridiculous things I have seen for a while as quite frankly, who's got:
a) a 4ohm cab, very rare
b) wants to wire up 4 8ohms cabs together to get two ohms.
c) would run their amp at a lower power setiing, but still want to play through a stack.
OK, now i've got that out of the way...
Mismatches upwards should be safe, ie, putting a 16ohm cab on a 8ohm setting.
Overally I'd say, just get an 8ohm cab to be on the safe side