We usually arrive in the morning, so they've been pretty good about getting a room before 4, but I agree with most of the previous posters, that having the room ready by 4:30 pm is not complaint worthy. The only time it would be is if you were at the desk and they promised you it would be ready by 4 PM and then it wasn't. I think without an implicit promise of it being ready I wouldn't really see a right to complain until maybe 5 or 5:30 PM.
As others have said, if the worst thing that happens all week is a 30 minute late check-in that's a pretty good stay.