I am not sure what hemp bedding is.
I personally will fill it with a good 6 inches of peat moss or dirt.
I would put my chicks out with out added at those temps.
Welcome.
The coop can not hold 4 chickens safely.
Have you already bought it?
Planting grass would be a waste of time because it will be gone in a matter of a few days and won't grow back.
If it is summertime you can probably put them out today.