I've been to SK for two weeks and Japan for three weeks. To be honest, I could have done better in SK (but I was basing myself around seeing a friend who lived in Daegu) but SK didn't grip me. I even struggled in Seoul to find things to do. Went to Jeju island too, it was nice, but again, but didn't feel the need to stay longer.
Japan on the other hand is a whole different kettle of fish. I did a lot in three weeks, but could have definitely done with more time there, including spending more time in each town/city I visited... it really was a flying visit to get in as much as possible.
Could easily kill four weeks in Japan, so that's my advice.