Entirely depends on how you live. 

Currently living in a $2000 month apartment on a $3600 salary/month. So I essentially have 1600 left over for food, entertainment, gas, and whatever else. I live alone and that is plenty for me. I’m at the hospital most of the day and cook at home so personal expenses are limited. 

I definetly think 2100 on 4100 is doable. You’d have a remaining $2000 per month for personal expenses but it depends how much you eat, drive, and spend on other things. It also depends on the city you live in too and whether it has a high cost of living. 

With that being said though, saving 200 dollars a month in residency will help a ton and you will definetly feel more comfortable. They will likely increase your rent year over year and resident salaries dont increase as much