On this video
https://www.youtube.com/watch?v=-uihfyMxuHs it is shown how to make a video with 50 fps from a video with 25 ftp. With 1080p video, it works. But if do it with 4k video, it doesn't work. I've tried doing in x264, x265 and ProRes, on the output video with 1 frame.
Can there be specific settings for 4k?
Only thing I can say to such vague info:
You will have to use either 64bit Avisynth or 64bit Vapoursynth otherwise 4k video will cause 32bit Avisynth to fail due to memory limits.
Cu Selur
And how to use 64bit Avisynth or 64bit Vapoursynth in Hybrid?
Vapoursynth is always 64bit, but for Avisynth the default is 32bit (since some of the filters are only available for 32bit).
You can switch between Avisynth 32bit and 64bit, by switching "Config->Internals->Avisynth->Avisynth type". (this only affects jobs created after the switch)
Btw. Vapoursynth usually is faster on systems with large core counts, since it normally scales better than Avisynth.
Cu Selur
Indeed, when choosing Vapoursynth 10 minutes of 4K video is done in 22 minutes (from ProRes 25 fps to ProRes 50 fps), and when choose Avisynth 64bit it is done in 2 hours 30 minutes. Thanks.