M.P.
This sounds like you set your 4 yo daughter up for failure. Why couldn't she have the candies now? You knew they were out easy to eat. I suggest that you making her wait did not make sense to you mil and that you have it in for her so she feels that there's no use trying to please you.
The issue here is not your daughter eating the candies when you told her to wait. The issue is your relationship with your mil. I suggest that your husband didn't back you because your focus was unreasonable. At 4 you cannot expect a 4 yo to recognize that she has to do what mom tells her when grandmas says it's OK.
Here is another situation in which it would be helpful to ask yourself the question, "would I rather be right or be happy?" Some battles just aren't worth fighting. You're moving. You have a solution to the mil problem. Why fight it now and why bring the whole family into the fight?
Later: Your husband isn't moving with you. You're facing a lot of stress. I suggest that your husband and you need to go to a marital counselor. I also suggest you read about non-violent communication. It's a way of talking with each other in a loving and accepting manner so that you can hear each other. Here is their web site. http://www.cnvc.org/learn/nvc-foundations There is also a book.
I suggest that your husband is in a tight spot between you and his mother. Find a way to stop putting him in the middle. Talk with him in a calm, loving manner telling him with I statements how you feel. Stop blaming him. Recognize that he's also stressed out and having difficulty finding a way to be with both of you at the same time.