My first impression was a bit of an overnute or slight ph problem. But after reading the form...2 litres of water per week in a 4 gallon pot is not nearly enough. My suggestion is to add two litres of nutrien solution (your half-strength mix) twice a week, and water twice a week with no nutes, with plain ph'd water.

A $7.00 aquarium ph test kit is usually much more accurate than the soil probes or test kits.