Responder nos comentários do OP:
@echo off
setlocal enableextensions enabledelayedexpansion
set inputfile=variables.txt
set tempfile=%random%-%random%.tmp
copy /y nul %tempfile%
set line=0
for /f "delims=" %%l in (%inputfile%) do (
set /a line+=1
if !line!==4 (
echo WORDS YOU REPLACE IT WITH>>%tempfile%
) else (
echo %%l>>%tempfile%
)
)
del %inputfile%
ren %tempfile% %inputfile%
endlocal