If any answer raises concern, avoid the link.
If you route your video through an audio system (such as a 5.1 channel receiver configuration controlled by an ANU088 style system), the receiver must feature 4K Pass-Through . If it does not, connect your video source directly to the TV and use an optical cable or HDMI eARC for audio.
Let me search for "S1 No.1 Style 4K" to see if S1 generally offers 4K content. 0 mentions "4K" for SONE-425. So S1 does offer 4K titles on FANZA. It's likely that SONE-088 also has a 4K version.