I recently found out the answer to my question about exit number four in the underground walkway!
One of you readers kindly asked the City of Sapporo about exit number four, and then explained it to me.
It turns out that exit number four will be part of a new building. It is still under construction, so the exit is also not ready yet. When the new building is finished, exit four will be opened. It’s not related to bad luck afterall.
Thank you very much for letting me know!