Normal is between 4 and 7, average of that being 5.5. Below 4 it's possible you can get what's called a hypo which would give you the symptoms you are describing.
I've never experienced this personally but i've learnt from others that have, that a banana for example would bring the level up as it's high in sugar.
Maybe others here would tell you better than i can !!!!!
Hi sarah 4 should be fine but if that is one of the first times you have been that low you are possibly going to suffer from hypo symptoms. Hopefully your system shorted itself out ok.