One of my girls has started to develop brown patches at the top of the plant in the 4th week of flower. Only seems to be one girls so far (out of 9). Soil PH seems ok and Im pretty sure its not nute burn. Appreciate any thoughts
Nothing to worry about really, since you mentioned that the brown spots started at the top then it's a clear sign of calcium defiency, and the pictures show it as well.
Nothing a high suffer of cal mag won't fix.
Hope it helps and happy Growing.