If you for some reason have a very long flight delay and don't think you can make it, you can change the ADR to something else on the
MDE app on your phone... you can just move it way out into the future day of and then cancel that... and it will open up a last minute ADR for someone else if they are looking, rather than just missing it and paying the fee. I landed at the airport at 10am and was in the MK and through security by 11:30 so I'd think that as long as a delay won't get you there past 2pm you'd be fine to make your 4pm ADR.
If you are worried about not getting another ADR if you miss your arrival day one, hold the 8pm for a backup and cancel it once you know your arrival day has worked out.