This is completely incorrect.
Adding a 8 ohm cab would put your total ohms at aprox. 2.6 ohms and adding a 4 ohm cab would drop it to 2 ohms. Adding more cabs LOWERS the resistance, not increases it. I don't know if the B200 is rated at 2 ohms or not, but if it is you could run either an 8 ohm or a 4 ohm extension and you'd be just fine.
That's true if the speakers are connected in parallel. If the speakers are in series the resistance would add. 8+4=12. Although I'm not sure why you would want to series two cabinets.
Cheers,
Jim

