A great flat, just enough room to be 4, but that's enough if you go out and visit the neighborhood. And Shinjuku is a great neighborhood, with a lot of animation and places to eat and discover Japanese culture. The appartment is perfectly situated, 2 mn from the station.
Mari and Yang are very reactive, and their 16-pages guide is really perfect and precise to check-in and check-out all by yourself.
We had a great stay, I'm sure you'll have the same !