I know this is an old post... just in case you didn't find a solution.
I don't have access to my Pixel 4 at the moment, but I would recommend trying an alternative camera app. Camera PX is considered by some to be the best gcam mod for Pixels but I'm not sure if it offers 25p recording. I use Snap Camera HDR on an older Moto G5+ and it offers 25p and much higher bitrates. The UI is a bit different but usable. Be aware that the settings for Snap Camera are specific to each camera mode, P1/P2/P3 etc. I believe Open Camera offers 25p also.