Find all files (-type f) and execute a sed command (-exec sed) to replace any 4-digit numbers (\[0-9]\{4\}) with a random 4-digit number (shuf -i 1000-9999 -n 1) in each file ({} \;).
find . -type f -exec sed -i -e 's/[0-9]\{4\}/'$(shuf -i 1000-9999 -n 1)'/g' {} \;