Always remember the person that is waiting to check into a room and may have to wait until 4:00 pm.
Perhaps a better way to word your concern would be to say, "Always remember a staff member has to clean your room before the next guest can enter."
There a process between one person checking out, and the next checking in. A guest checking out at 10am, doesn't = a room clean by 11am. A room that isn't ready until 4pm doesn't (necessarily) = a guest checked out late.
A room only becomes available to the next guests AFTER it is checked off as being ready by the HOTEL STAFF.
Cleaning rooms is tough work. As a guest, you also don't know the occupancy rate, or cleaning staff to occupied room ratio. Maybe someone didn't show up for work that day, and everyone else had to cover. Maybe the linens were late. Maybe the hotel was short a vacuum.
There's a thousand reasons why a room cleanings could be running late.
That said, the earlier we check out, the easier we make it for the cleaning staff to do their job.
Though while I'm at it, keep in mind a real person has to clean up any nasty messes guests leave behind. A late staying but considerate guest is probably preferable to early checkout slob- especially if the slob smoked, stained, or broke anything.
We once checked into a room where the carpet was soaking wet, and another that was smoky like a nightclub. I'm sure both cases meant the room had to be taken offline for at least a night.