I'm not sure if it works that way because it also vary bitrate but based only on pixels per second
4K resolution is 3840 × 2160 pixels = 8.294.400 pixels x 30 frames = 248.832.000 pixels per second!
1080p resolution is 1920 x 1080 pixels = 2.073.600 pixels x 60 frames = 124.416.000 pixels per second!
720p resolution is 1280 x 720 pixels = 921.600 x 120 frames per second = 110.592.000 pixels per second!
well i won't even say about 360p. (remember that 8x slow motion is recorded on 360p)
So based on this, we can clearly see that the sensor itself is doing half the job on slow motion mode that it can do on 4K.
Anyway, can we actually change the resolution recorded on Slow motion mode ? is that on the camera APK? or where? can someone tell?