hi,I had the same problem here,as even my affections to Shika is already 25,but the H- event is still locked. And there's no more hints about the fourth unlocked event.
So I wonder if that a bug maybe due to the wrong unlock order. Because I'm not very interesting to Shika so that's my last one locked H event of every girls.
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Problem solved. Just go to keiko's place in morning and night then the main line can go on. After that will be the Shika's fourth event.