By what % approximately does the RMR/BMR decrease in an individual severely restricting intake due to eating disorder (200-300cal daily for 4 weeks)?

No way to tell. Without data like existing starting weight and caloric need from optional activities like exercise such a calculation is impossible. Accurate measurement might be possible in a research lab. The practice could easily bring the person to a deadly range of blood electrolyte imbalance that could trigger an abnormal heart rhythm resulting in death.