D.B.
I say go see what's up with her. If she's waking at 4 there's a reason. 21 month olds don't just pop their eyes open at 4 to get you to let them cry it out.
She may be teething, have an upset tummy, a stuffy nose, had a bad dream, et cetera.
Whatever the case, she needs to be tended to - especially if she's sick.
Perhaps the book, "The Vital Touch" by Sharon Heller will help. Check your local library for it or grab it off of Amazon.com.
Good luck.
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