Ouch, that's rough being that you are 4 weeks in!! How much of the plant/s are affected already? Neem oil might be safe to use at this point but it's pushing it since it has a terrible flavor and odor and you are 4 weeks into flowerng. It will ruin the final product if applied too late into it. I've heard of people using coriander oil, even then you'll want to avoid the buds. Best bet is to cut any fan leaves that are affected and pitch them FAR from your operation. Increase the the air circulation and drop the temps a bit, maybe add some humidity. Remove any webs you see with a vaccum. You can wipe the undersides of the leaves with water (not during lighting wait till your dark cycle)
I also read of peole using pyrethrin bombs but I'm always leary of stuff like that.
EDIT: After harvest be sure to disinfect the are before growing again. You can safely use neem oil then and pyrethrin bombs set about 2-3 days apart. Clean ALL equipment good! Those little fuckers are hard to get rid of.