99.999999% of the time - the Tokyo flights are on time. There are very rarely delays like you hear of in the US for these mainline, business heavy flights. They're very reliable. That being said, if you're worried about any problems - just book OZ into ICN at 11:20. There's tons to do in the airport - free showers for connecting passengers, Korea Traditional Culture Experience, shopping, good food etc. Free nap room too.

You can also fly KE if you want, they check bags to other airlines even across alliances: https://www.koreanair.com/mobile/glo...-check-in.html

Personally, GMP and HND are much better airports for their simplicity and ease of access - but if you're worried about a layover issue, just stick to ICN. The MCT for GMP-ICN flights for international by the way is around 3 hrs for both KE and OZ. So 4 hrs is technically more than enough.