As others in the thread have already noted, using the fifth night as a guaranteed extra, extra late checkout works well. No need to formally checkout online on after the fifth night. The hotel will automatically checkout you out on the last day. If you want to verify your folio before you leave, just stop by the front desk and ask to review your current charges. That is a very standard request and won't raise any flags.

TL;DR Use the fifth night for an extra late checkout and leave whenever you're ready. Hotel will automatically check you out at the next day. Get an interim folio from front desk before you leave if desired.

--Jon