I've had success loading the occasional 4k mp4 that otherwise wouldn't load, simply by changing the extension to avi. The ffmpeg option didn't do it. If those options don't work then Shutter Encoder is your friend.
Also installing LAV video decoder package introduces directshow decode filters to Fusion.
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Asus Prime X370-Pro+R7 3700X@PBO+32 GB G.Skill AEGIS DDR-4@3200MHz Sapphire RX6700 10GB Adata A400 120GB System,A2000 500GB Scratch SSDs Media storage:"Always in motion is it" BMD Mini Monitor 4K Windows 11 Pro+Resolve Studio 18+Fusion Studio 18