Hi,
There is some evidence that severe hypos can, over time, cause harm.
However, I am not aware of any evidence suggesting that being in the 3-4mmol/l range causes any complications at all.
Many of the non-diabetic population spend a great deal of their time dipping in and out of the 3s during normal life, and it isn't considered a hypo for them (because they are not on strong medication that may drive the blood glucose lower, such as insulin or gliclazide), so I would imagine that there has been no research done on this in non-diabetics.
You may find these two studies interesting
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2860007/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2711670/
But please remember that there is a vast difference between dipping into the 3s, and what is referred to as severe hypoglycaemia, which is much lower than that.
Edit to add:
I see from your posts
@drew900uk that you are only recently diagnosed, and there was some question as to whether you were T1 or T2. It would help members to understand your queries if you were to update your profile with your latest diagnosis, which would then be displayed under your profile picture on most screens. Many thanks!