Mario Kart 8 came out on the 29th of May in Japan, 30th in NA and 31 is Aus. Just looked it up. Where did you get "Mario Kart had 4 days in Japan" from?
From counting? The PR from Nintendo included the full weekend, Media Create/Famitsu numbers go through the 1st, VGC numbers go through the 1st (in Japan).
29, 30, 31, 1. Four days.