Weird spots on leafs

Without pictures and no diary, answering this question is impossible. Quite unlikely to have a calcium deficiency if growing in decent soil after only 4 weeks...........if you are growing in soil that is............but without a diary, who knows what you may or may not be doing?

You should buy CalMag as an additive! You have a calcium magnesium deficiency and can be easily fixed with any CalMag product out there. But be careful with it, it’s very potent. So start out with maybe half the dosage the manufacturer tells you to feed. They usually say 1ml/L. Give it 0.5 the first week then 1ml/l if it doesn’t get better. Calmag can also be sprayed in the leaves to prevent new rust to come! But won’t heal the damaged leaves. Because if you feed too much your plant will burn because of overfeeding. Good luck my friend 😃

If there is no interveinal chlorosis (paling between veins on leaves) coinciding with the espots, i'd wager you need a little more Ca in your fertilizer. Pictures would help. try to up your ca with minimal impact to the levels of other nutes in your fertilizer.