hmm I have a lot of questions to help you out as best I can. Your using soil it says, what is in your soil did you mix it yourself? 7.5-8 pH is a bit high for cannabis. Your aiming for lik 6.0-6.5, But keep in mind if you are using a living soil then the biology keeps your pH where it needs to be assuming you have a proper soil mix. Your plants look a bit small for 4 weeks. Lets get your soil where it needs to be
ppm of 1500 is pretty high, is this from your run off? are you feeding your plants?
like 1