K.R.
Give her Benedryl. It's safe and it makes them drowsy. You're brave - 4 1/2 hrs. There's NO WAY I'd travel without Bendryl for that long of a plane ride with an 18 month year old. One thing to think about it is trying the Benedryl beforehand on her, b/c it sometimes can have the opposite effect and make her hyper. Good luck!