Is it possible for a person to lose 100lbs on the atkins diet in 4 months if they have over a 100lbs to lose? staying under 10-20 carbs a day?

Unlikely: To lose 100 lbs in 4 months, you would have to take in about 3000 calories less per day than you are burning up (1 lb= 3500 calories). Unless you are currently taking in well over 3000 calories daily and are able to make changes in your diet to eliminate at least 2500, then do exercise to burn another 500 calories daily, it is not possible to lose that much in 4 months (and probably not healthy)