L.,
Both of the original responders (Judy and Tina) have it right on! Adjust her schedule a little each time. We start a month out and make 15 minute changes each week, so by the week before they are going to bed an hour before or after their normal bed time so when the time changes, they are still on schedule after the change. They don't notice the difference and if you don;t say anything, they don't even know it's happening, unless they can read the clocks. My aunt still does this in her home with my cousin who's 17 now. She's been using this method even for herself before he was born.
Should you not have 4 weeks, just adjust the amount of time each week remaining to even out to an hour for the week before. If you are left with only a week or so, then do 10-15 minutes each night.
Hope that helps.
Blessings,
Rolinda
Wife, Mother, Friend