When you say your ppm is 300, is that the ppm of your nute solution when you water the plant, or is that the PPM of your hempy reservoir after a day when your EC is 4.5? If it is what you are putting in, what is the EC of the reservoir water the next day when pH is 4.5?
When pH drops, it can mean nute concentration is increasing. 300 ppm is not a lot, so if you are putting in 300 ppm, then chances are your nute sol is increasing concentration due to a salt buildup in your media, and the perlite should probably be flushed.