Nitrogen def? I'm using magne cal as the local shop had none of the other. They are in soil and 4 weeks old
At 4 weeks, without amendment to the soil, there usually is a depletion of calcium and/or magnesium when using city tap water. Now is the time for the addition of Calmag, or a similar product into the nute formula. Some people say this is a waste of time in soil. Maybe real "dirt", but using potting soil, it needs help. PH also could be an issue, as it will "lock out" nute intake, especially if it is low (below 6 in soil) You have to stay in the 6.2 - 6.5 range for optimum uptake of your nutes, unless you are organic, then it is self regulating. A flush might not hurt, afterwards, start with a low PPM and build back up to your norm (in soil I would stay at no higher than 700 PPM, for an average plant. I have a Cotton Candy Cane growing now that burns at anything above 650 PPM, but I have other plants that kick ass at 900-1000 PPM in soil.
Me, myself would test soil runoff to make sure your PH is OK. That is where I would start. If you flush, take 2 pieces of wood and place them across a 5 gal bucket. Put your pot on top of the wood, run a mild nute formula thru your plant until you have filled the bucket (do a PH test then, and I will bet you are in the 4-5 PH range. If it is way out of wack, use hydrated lime which you can get at a garden center, and adjust your feed solution as needed)