The pH of my solution falls to 4 a few days after every clean and refill. How can I raise the pH and stabilise it?

A pH that drops is a sign that there is too much microbial life, not necessarily pathogenic, and dying microorganisms acidify the solution. The simplest way to raise your pH level is with tap water, if it is of an acceptable quality. Otherwise, you can add pH +. Avoid excessive fluctuations in pH and EC.