I don't know how Depo affects weight loss, but for birth control pills, I have read that the amount of estrogen can have an effect, and part of it is due to water retention. I can't remember whether it was the high or low estrogen that was better, sorry. If you do a search on Google, you'll probably find some good info.
By the way, 4 pounds in a month isn't slow, especially for your weight. Look at weight loss in terms of percentages instead of pounds, and that will put it more in perspective. You're doing much better than you think!
