When I bought my Bass 400 in 1991, I phoned asked Mesa if I can run two Mesa cabs at 2Ω load. They said no problem (Michael Anthony used to run his that way too). So for 20-someting odd years I ran it that way with no issues. I even had the head in at the Mesa Hollywood store once to get a check-up 'cause I was in town, it was clean. I also ran my old Mesa D180 this way too. Never a hiccup with either head.
But I agree with the other guys that say if you're buying two 410 cabs, just get 8Ω ones. You already have a 4Ω one for when it's needed.