The fourth lock was a deadbolt on the outside.
In the news, the reporter states that the transit authority reopened the Interstate H1. However, Hawaii does not have a transit authority and the freeway is managed by the State Department of Transportation.
When Danny is talking to Grace, wet spots from rain appear and disappear from his shirt.
After the young girl is placed in the room with the older girl, we hear four locks turned on the door, but there are only three locks.