When does your flight leave? And how do you plan to get to Narita? By train, or flying from Kansai International or Osaka Itami airports?

OP says 16:00 or 4pm. That's plenty to time to get to NRT by plane or train. Though flights from Osaka to NRT either arrive before 10a or around 4p, so not attractive. Flying to HND requires a long transfer between the two airports. Probably just better off training the whole way.

The thing about taking the train day of your international flight is that it adds so much travel time. The risk of you missing your flight is minimal thanks to the reliability of the trains...they stop for earthquakes, strong wind (like typhoon) or heavy snow and "accidents" and otherwise run exactly on time.

From Osaka proper, you'll need to change trains at least twice - once in ShinOsaka and once in Tokyo or Shinagawa - to get to Narita Airport. It's not like it's hard, but can be energy consuming just navigating strange places with luggage.

You could send your luggage to Narita Airport, but you need to do that at least 2 days ahead of your flight.

If it were me, I'd spend the final night in Tokyo and enjoy the morning doing some shopping or sightseeing. But that's me.

Good idea to spend your last night in Tokyo. The trains are reliable and are not the problem. The problem for me would be the added 4 or 5 hours transit time when you are already facing a long flight home(10 hours or more) if going back to N America or Europe.

Aloha!