So I received the reply below from Pico Support. Could someone from DEO VR comment on whether your app changes the default "rendering resolution" on the original Pico 4 when watching a 1920p or 2160p video stream? If not, then the Pico 4 Ultra version of your app should indeed have better video quality. If it already increased the rendering resolution on the older device then it would seem unlikely that the Pico 4 Ultra version would look any better.
[From Pico Support: While watching 2160p or 1920p videos on the YouTube app with your Pico 4, you are indeed actually viewing videos at a rendered resolution of only 1500p since the resolution from the original platform is not the same that the device could reproduce and it's limited to the technology of the lenses, software, etc.
With the Pico 4 Ultra, due to the faster processor, the video playback quality got improved to 2160 × 2160 (× 2), 1200 PPI (pixels per inch) with a rendering resolution able to achieve: 1920 × 1920 (× 2).]