Overall, really good place! 2 min walk to Otsuka station, which is 10 min away by Yamanote Line from Shinjuku and 20 min away from Shibuya.
We stayed 2 people, so the beds were big and comfortable (I don't know if they would be ok for 4 adults)
On our first impression, the apartment looked a bit old and we missed having essential cooking products. Also, there were not instructions on how to recycle, and Japan is a very strict country in that sense. We missed having wi-fi.
We had booked for different dates at first but we had a problem with our flights and wanted to change for 4 days later. Even though the apartment was not booked and our changed was asked 5 days after the initial booking, the changed was not accepted and we were partly charged for rebooking (we even booked for more days).
However, after 2.5 weeks of staying, the apartment felt like home, comfortable and easy to live. Nice neighbourhood, JR line and essential facilities are close. We would definitely stay there again!