Every Diabetic should make sure that their Bg levels don't drop below 4 mmol/l as below that is considered to be Hypoglycaemic, something to be avoided. I prefer my levels to between 5-7 two hours after meals then I avoid hypo symptoms which for me start at around 4.5 mmol/l.
If you are in the 4's and you feel OK then I wouldn't worry too much, however if you have no hypo symptoms when lower then your hypo awareness may be not good. Something to consider if you drive or operate machinery as low Bg levels can have a detrimental effect on dexterity, concentration etc.