S.H.
Has she ever watched all 4 kids for you by herself? That might do the trick! :)
Tell her yes, you chose to have those 4 kids, but you also chose the responsibilities that come with them! They are not an excuse, you just realize that you are responsible for 4 little lives that need you to take care of them! She needs to respect your choice & understand that you aren't free to do everything!
I don't think she will ever truly understand until she has her own children. Just stick to your schedules & keep telling her no when you have to. Do what you can with her, but don't sacrifice your family in the process.