type config.ini | Findstr /v "4:" >%temp%\1.txt
REM get the value of 4, suppose it is OFF now.
echo 4:off >>%temp%\1.txt
type %temp%\1.txt >config.ini
It will first take all values from your config file and store in a temp file.
then it will take the value of your config line and append in same temp file
now it wil take all content from temp file and save in config file.