You have to use a special cable because the outputs are combi-jacks. On the XT versions there not, there just speakons. The difference is that the speakons have 4 contacts - a +1, +2, -1, and -2. For normal use you connect to +1 and -1 and for bridged you connect to +1 and +2. with the combi jacks there are only 2 contacts for the speakon part +1 and -1. Thats not Matrix's fault in Neutrics. They dont make a combi that gives 4 connectors. Therefore in the GT version, because we wanted the ability to run 1/4" outputs, we have the combis. To make the bridge mode work you need to connect the +1s from each output. Bit of a pain - but necessary if you want the 1/4" o/ps.
So yeh - basically any speaker cable (that should have 2 individually covered cores). Split the cores and stick one speakon plug on each core, attached to the +1 connector of each plug then connect one plug to the L and the other to the R O/P, with the selector set to bridged.
For your cab you'd need to run in 16 Ohm mode as the 4 Ohm is too little when bridged and the amp will go into protect. So - basically you have the choice, of running a single channel in to your cab configured at 4 Ohms - for 400W output OR bridge the amp into you cab configured at 16 Ohms for 520W (ish) power.
What is the power rating if your cab? if your using V30s and similar rated speakers you cab is 240W - so id stick to the mono channel at 400W anyway. If your running higher rated speakers and have a 400W+ cab then it may be worth bridging for a little headroom.
The safety thing lightningboy refers to isnt so much to protect your speakers though - if you run 800W into a Jack socket your more than likely going to melt it !! - its not built to take that high power - hence speakons on PA equipment as they are. The last thing you want is you jack socket welding itself to your amps output sockets.