Gaining

So im 6"0 and 215. I lead a very sedentary lifestyle. If I eat 4k+ calories per day, what would the expected weekly/monthly gain be?

3kg per day

You must be joking! On the basis of the OP requiring 2000 cals per day (his BMR) and taking in 4000, that is 2000 excess calories per day. 3500 excess cals produces a pound of fat so max gain would be around 1/2 lb per day or 3 1/2 pounds per week.

Assuming a typical sedentary burn rate of 12 cal per lb of current weight you need 2580 cal / per day to maintain.

Assume 100% calorie retention and you have a surplus of 1420 calories per day and 9940 calories per week.

9940 calories per week at 3600 calories per lb gained is 2.7 lb per week or 1.25K.