^I'd prefer the Neem at this stage to the soap. The soap won't kill the mites. And 4 weeks is a long time and they will take over if you don't act. There are a lot of natural recipes posted here to like DaMagoMan said. Check out the search engine above.
Cooler temps and higher humidity will help slow there growth.
One of the first things I cover when they are deteted.
The soap will kill 'em !!!
ex-specialy it he treats them evey 12-24 hrs.
You will also see some leafs die, the ones with the largest number of mites.
It's not a ...... damn , what's the words I'm lookin' for ...
matter of what to use at this point , it's a matter of What can he use and not hurt himself ...
the soap not Neem , Trust me I have experimented with myself.
Never use Neem with in 3 weeks of harvest But If you do,
mist/wash down the plants twice before you harvest.
on jorge cervantes ultimate grow dvd he advises you dont use neem 10 days till harvest, im using it at around 14days til harvest right now, and plan to just spray the remainder off with water until harvest.