Hi nybrid
You aren't on insulin, just metformin so I wouldn't worry about low 4s and 3s.
I often get low 4s, 3.2 a couple of days ago..... The more I get the better, but I am no longer on metformin. My consultant advised me to stop the metformin, he said it would do no harm, but no good either. With BG levels you have I would come off the metformin.
My hospital records have me as a T1 (high GAD), I understand the dr's only recognise two types, T1 and T2. But like you I am not on insulin, thus in the honeymoon period or T1.5 or LADA.
I contacted the DVLA over a year ago, I have a letter from them stating ok to drive and to contact them again when I am on insulin.
My advice; carry on with the low carbing, but be prepared to have to go on insulin some time in the future... The longer you can keep your BG low (below 5s fasting) the longer you (and I) will keep off insulin.