Type 2 with 'true' hypo?

I don't think there is anything to worry about with the odd lower reading. Maybe it means his blood sugar levels are becoming more normal, maybe he ate a fewer carbs or did a bit more exercise the day before. Maybe he is becoming a little less insulin resistant. 3.9 is on the low side of normal but 4mmol isn't a figure below which it's immediately dangerous. (in places that use mg/dl the level of 70 or even 65 is given for hypos. Like 4mmol, round figures ...70mg/dl converts to 3.88mmol/l ) .
Non diabetics also go below the 4 level, in response they usually eat! If your husbands levels are low on a frequent basis then perhaps he may want to speak to his doc about reducing medication.
When his glucose levels fall ,his system should, in any case, stop insulin production and if it's really necessary his liver will release some. Eating just a small amount of carbs will however raise his levels a bit and make him feel better
There is more potential danger for people on insulin and sulfs. In the case of injected insulin it is already in the body and can't be controlled or stopped so bg can drop to dangerously low levels . Even on insulin, if my levels are where your husbands are and it was some time since my last rapid insulin I might only need to take 4g carb to bring my levels to acceptable levels.

There's a documented phenomenon whereby established diabetics "lose" the threshold for hypos and start to experience hypo symptoms at higher thresholds than the "routine" 3.9 for example, so even at say 5, they can have hypo symptoms, even though it shouldn't happen at 5. Sorry but off the top of my head I can't recall what it's called, you'd have to dig about the web or ask your doc.

You should get on it straight away because when I read about it before, the info said that over time, established diabetics having hypos at higher than normal levels, can eventually start to also lose the impending warning signs of hypos and they can lose consciousness without the shaky feelings etc that they have gotten used to as the warning that they are going low. I can't comment on the accuracy of that information but that's what I read, I'll look around again for it when I get a chance, hope it helps, definitely a case of better monitoring and maybe ask for tests for reactive hypoglycaemia as well.