You can use the -exec option of find. Here, '{}' is replaced with the file name of every MP4 file. This will deal with all kinds of file names, even those containing spaces or newlines. You need to supply -maxdepth 1 to only search the current directory.
find . -iname "*.mp4" -maxdepth 1 -exec mp4box -inter 500 '{}' \; An alternative, more convoluted way would involve piping the output from find into a loop with read. Here, every file is delimited by the NUL character, and you need to tell read to split the input on this character, which is achieved by -d ''. You also need to quote the variable "$file", so spaces or globbing characters in the name are retained.
find . -iname "*.mp4" -maxdepth 1 -print0 | while IFS= read -d '' -r file; do mp4box -inter 500 "$file"; done