well, looking at the Carvin site, we see this:
http://www.carvinguitars.com/manuals/BRX212.pdf
which shows a few things.
1. The B800 has both Speakon and 1/4" output jacks
2. Shows that 4Ohm and 8Ohm cab combination is just fine
However, just keep in mind that there will be unmatched amounts of the amps total power going to the cabs. The 4ohm will draw more than the 8ohm. This may not be an issue in your playing situation, but just keep it in mind.
good luck.