Generally, 1-2 lbs is the weekly maximum for healthy weight loss. That translates into roughly 4-9 pounds per month. I like to think of it in terms of weeks though, because that ensures that you are losing at a steady rate rather than losing nothing during the first two weeks and then 9 pounds during the last two weeks of the month - does that make sense? Essentially, not more than 2 lbs per week is recommended.
I know that may not seem like a lot. But if you loose two pounds per week, in a year you could lose up to 104 pounds
that is A LOT! If you aim for 1 lb per week, in a year that can mean a 50 pound weight loss. Also, a bonus of losing at this rate (1-2 lbs per week) is that you will have time to adapt to whatever lifestyle changes you are making. And those lifestyle changes are what will keep the weight off after you have lost it.
However, I do want to add that I'm sure there are medical reasons for which some people must take it slower. That may be a reason why a doctor may recommend that, or the doctor may simply mean that gradual changes are probably easier on our bodies than consistently losing a lot. I always say that I don't want to shock my body with all this weight loss. Nice and easy does it
Did your doctor specify why he recommended 2-4 lbs monthly for you?