If the cab is 8 ohms, you don't want to use it alone with the SVT-CL You can get away with this at low volume, but you risk damaging the amp (and maybe the cab too) if you push it hard.
What kind of cab is the 4x10? Probably your best bet is to add a second 8 ohm 4x10 cab just like the first. This will get ungodly loud if you need it to.
