I have been advised by my Diabetic nurse to eat a standard size Mars Chocolate bar if my glucose level goes below 4
As Ken says, thats not normal advice today. Personally, a whole Mars bar would raise my glucose levels far too high but possibly not quickly enough.
Standard official advice is to use glucose (very fast acting) you can use tablets, liquid (lucozade), gel or 2 -3 teaspoons or 3 lumps of sugar if thats all thats available.
take 15g carb and wait 15 minutes, test, if blood glucose back to normal . Fine.
If it isn't and is rising only slowly repeat treatment.
If its a while to your next meal, then eat some slower acting carbs such as a small cereal bar, a fruit or a mix of protein/carb such as half a sandwich.
If driving wait 30 min before driving again, check bg level.
see :
http://www.library.nhs.uk/DIABETES/ViewResource.aspx?resID=59817
Now the confession,.Like most people I don't really like dextrose tablets, but they work very fast , and help stop you eating anything and everything when hypo. I always use them whilst exercising and if very low, but I I've found my own (nicer tasting) ways of dealing with mild lows, (such as the just under 4 you suggest) as I'm sure you will as you learn how much you need, and how ou own body copes with hypos.