Yeah I misspoke (misstyped) on my previous post. It'll hit 4000 within a minute if it gets a good restart, not 3000 as I had stated before. I get 12.6 also but I am running at 587. I thought about trying the mystery firmware but opted not to until I hear more results. My slow board will push 4150 or so after a good restart at 587. The thing is the other boards will do 4300 easy with higher freqs, but if I do that the slow board just gets stuck around 2700-2800 (like your screeny). I may well try swapping boards when my B4 arrives to see if I can build a higher freq unit and lower freq unit.
I would contact them before switching boards between unit's. It's would break the warranty seal and be a shame to lose warranty on two pretty new unit's over a little bit of hash.
Have you contacted them and said it did not fall in "12.93TH/s ±5%" of batch 2? If not do that before breaking seals and voiding warranty.
It gets 12.6 which is within the 5% range. I realize it will void the warranty. I wouldn't do it right away and I haven't even gotten the B4 so who knows, it might have 3 great boards that all get 4300 in which case there won't be any reason to swap boards around. Just something I would consider if it looked like it was worth doing.