As long as you set up a reservation, you can enter anytime while the park is open. I've gone the last few months for Milkyway photos and there has not been a 4 hour time slot for this park at all. I would check but I'm fairly sure the gates close at 8pm to all incoming visitors. You can however easily leave after 8pm once you are in the park. "Technically" you not allowed in the park after 10pm unless you have a camping reservation. I've seen quite a few folks there fairly late and based on the number of tents actually set up, I would highly doubt they made camping reservations so take that how you will. Milkyway photos have been fun over the last two months with fairly clear skies. July was the better month so far since it was visible for quite awhile and the sky was perfect up until Fredericksburg did their Independence Day fireworks.
You have to pick a time to come in the gate. Like a 4 hour window. That's just too go inside. Once inside, you can stay overnight but there's signs saying you can't sleep in your vehicle.
You can exit anytime. I did as what you say here.
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