If you are only four weeks into flowering, I'd say you're good for some neem oil or whatever you choose to use to eliminate them. Even if you're growing a very quick strain, you still have at least three weeks to go. I've sprayed later than that and had no problems with taste or aroma (and I'm a bit of a snob). Just do it early in their day, and raise you're lights up a foot or two. By the end of their day, they'll be dry and you can lower your lights again. If the mood strikes you, I'd even say that you're good to spray them five days later as well. This should stamp them out for a long time. Also, cutting you're fans back to increase temperature to 28 degrees or so for a couple of days, 10 days after the first spraying, will ensure that any eggs that may have hatched anyway will yield mites that are too sluggish to survive and procreate.
So thats:
1: Raise lights, spray plants early in day, let them dry, lower lights
2: Five days after first spray, repeat application
3:Ten days after first spray, increase temperature in room to 28 degrees Celcius for two days
4: Reflect on the fact that all those mites you killed really only wanted what you want, to suck on tasty cannabis and f%$#. NOW SNAP OUT OF IT! EITHER YOU"RE WITH US OR YOU"RE WITH THE SAPSUCKERS!