4 hours from now
28 hours from now
This one doesn't require me to know the current date and also works as a sanity check to make sure I'm not confusing AM and PM.
"Why are you asking me about number of hours, Siri?! I just said tomorrow, damnit!".
This doesn't guarantee to come up with the best implementation.
Do you always naturally know the date? Personally, I don't.
I will, however, always know how long 4 hours are, even if I travel to another planet someday. It also has the additional benefit of working as an AP/PM check.