Hey @ Sjake72!
You got a calcium deficiency over there. Anyway here are few tips to improve your growth:
1.- Humidity: 40% is quite low for 4th week of grown. It should be around 60-75%
2.- PH: Under hydro systems you should stay between 5.5-6.3 wich is OK. But you should stay over 5.7-5.9PH on this weeks and go leveling up on the next weeks till you arrive 6.3 on the 5-6 week of flowering.
3.-EC: Having 1.56EC is quite a lot for the 4th week of growth. Stay around 0.9-1.0 as much.
If you need further instructions feel free to contact me trought the chat