How much K-meta & sorbate?

I have 4 gallons of cider going and I am ready to stop the ferment. I just don't know how much of each of these chemicals to use. Please help.

Simply adding sulfite and sorbate probably won't halt the fermentation, there just too many yeast cell at work. You need to reduce their population somehow first. Cold-crashing the entire batch and racking off the yeast cake before adding the stabilizers seems to be a pretty popular technique. The clearer you can get the cider, the fewer yeast cell will be in suspension.

Unless the manufacturer's direction say different, shoot for 1.25g/gallon sorbate and 50ppm Kmeta.