FFMPEG doesn't work with mp4 and stdout

You can't use MP4 with STDOUT. The MP4 container requires the encoder to go back and make changes to the beginning of the file after it is done writing through the end. Since STDOUT cannot seek, outputting media in an MP4 container is not possible with STDOUT.

You can use the empty_moov example ffmpeg -y -framerate 0.5 -pattern_type glob -i '*.jpg' -f mp4 -movflags empty_moov - | ffprobe -

-movflags empty_moov

Write an initial moov atom directly at the start of the file, without describing any samples in it. Generally, an mdat/moov pair is written at the start of the file, as a normal MOV/MP4 file, containing only a short portion of the file. With this option set, there is no initial mdat atom, and the moov atom only describes the tracks but has a zero duration.

This option is implicitly set when writing ismv (Smooth Streaming) files. https://www.ffmpeg.org/ffmpeg-formats.html