Rachel.
You do not state if you are a Diabetic......? Your profile tells me nothing.
Your blood sugar levels will fluctuate throughout the day in response to food/drinks you consume.
4.9 mmol/l is not something I would usually be worried about in any circumstances apart from the fact that for me I may be dropping down towards a hypo zone.....too low, I get hypo symptoms around 4.5 mmol/l. All depends when it was taken really.
We cannot diagnose here and if you are not Diabetic and you have concerns about anything then please see your GP and discuss this.
Ken