Yes, the slave is for you to run your Boogie into an external power-amp or amp head. It's the signal from the Mesa's pre-amp.
If you are running two 8 ohm cabs from one amp, the load will be 4 ohms, (two x 8 ohms = 4 ohms). So that is why it is written that way. It is written that way with the intent of using either two 8 ohm cabs out of those jacks, or a single 4 ohm cab into one of those jacks. Boogie's are not really sensitive to load mismatches though, so you can fairly safely run a single 8 ohm cab from the 4 ohm jack if you wanted to, but it will wear on the tubes a bit more.
