It all depends on whether the 3.9 is falling further or at the bottom of the valley and on the way up - if you have lost your hypo awareness you can probably funciton fine at 3.9 but, and this is the big but, if you don't feel any symptoms until you're in the low 2s it can hit you quite aggressively, so I would err on the side of caution and treat it at 3.9 (not too much!) to bring it up a little.
Interestingly, when pregnant you are told that you're not hypo unless you're below 3.5 and that 3.5 -4 is acceptable. I found this hard to understand, surely you are either hypo or not whether you are pregnant or not? Anyways I certainly wouldn't drive or do anything requiring any concentration with a blood sugar under 4.