hi，magicsmoke,
Glad to receive your question，and I can understand what you’re confusion.
Now let me answer your question：
In your command , the encoder used is h264_omx.But VPU didn’t support h264 encoding,so you can change h264_omx to hevc_omx.
ffmpeg -i /dev/video4 -c:v hevc_omx test.mp4
And then you can get test.mp4,you can use ffplay to play it on the desktop
ffplay -i test.mp4
I hope this reply will help you