<3.5 is technicaly hypolgycaemia. So your 4.5 is absolutely fine and I would not be worried about that at all if I saw that myself mid ride.
I let mine elevate, ideally less than 10 pre ride by lowering my insulin pump rate, testing regularly and having small amounts of carbs regularly, typically 20g/hr. I've seen readings as low as 2.9 which theoretically should have me on the floor, buut whilst exerciseing give me no other symptoms in addition to the normal sweating, elevated HR and appropriate breathing rate. Non-exercise related hypos are nasty and give me the shakes, light headedness and mental confusion as well.
I'm LCHF other than during exercise as well, adn try to keep my HR<145 for steady efforts, let it get up to about 160 on the hills. One thing you could try, which a lot of keto adapted athletes find is that you need a little carbs during exercise. As I understand it you neeed it to keep both fat burning and carb burning metabolism in parallel, although the fat burning can predominate with suitable training which it sounds like you have.