The DroidCam - OBS plugin integration now includes support for 4K video.
This option is hidden by default.
Before trying 4K, please consider the following:
Most phones have special optimizations for recording 4K videos to internal storage. While DroidCam is also heavily optimized - the local recording optimizations do not apply to it. In other words, even if you device can record 4K, it may not handle 4K transfer with the same efficiency and will be under more load than usual.
4K (3840x2160) is 4x the size of 1080p FHD (1920x1080). This means 4x more data to process for each frame.
At 30-FPS, thats 120x more per second.
Most live streaming platforms, and most teleconferencing software,
are limited to 720p or 1080p for similar reasons.
Using 4K video at the input (and having it downscaled to 1080p) has negligible benefits, all while adding even more processing cost.
In most cases, 1080p is more than sufficient for producing great content.