Merge audio and video in a monitored folder
Creating target files in the watched folder is asking for trouble, so I would create them "outside" instead:

#!/bin/bash
now=$(date +"%d_%m_%Y_%Hu%M")
cd "/Users/tomvanwinkel/Documents/Convert/Merge"
mkdir -p ../Target
for filename in *.mp4; do
    stub="${filename%.*}"
    target="../Target/${stub}_ok_$(date +%Y-m%-%d_%H)"

    /usr/local/bin/ffmpeg -i "${stub}.mp4" -i "${stub}.wav" -c:v copy -c:a aac -map 0:v:0 -map 1:a:0 "${target}.mp4"

    rm -f "${stub}.mp4" "${stub}.wav"
done