As Mo said, 4.5 is OK, but if you've been consistently running much higher then you may feel false hypo symptoms just due to the drop. I get the opposite effect as I normally run <6, so if I'm up at 9-10 I feel ******.
I would say keep an eye on it adn if you get below 4, have something to eat, a biscuit or two, maybe an apple. Track it though the day and if it's going consistently low, then maybe call your GP.
Question is has anything changed in your diet or other factors recently? I have to be careful with my porrdge dose as it's slow release and I can get a post meal drop myself.