If Charlie wakes up and it's still 4:30 or 4:45, then I usually wait a little while to see if he can get back to sleep. The only exception is if he wakes up absolutely bawling. Then I will go get him right away.
Since I adopted this policy, I've been pleasantly surprised. Sometimes Charlie will get back to sleep fairly quickly, and then sleep an extra 3 hours or so! When he does that, I go back to bed too and my own sleep feels so much sweeter.
If your LO wakes up before a certain time, do you wait and see if they'll go back to sleep?