Please put the code below in a bat file, place it in directory with mp4 files. Before running real renaming, please remove "echo" before "move". please be carefull with renaming bacause (theoretically) it is possible to have same name for different files.You'll be prompted to confirm if you want to override the old one.
Code splits each filename after dash and renames the file taking first item. Good luck.
@echo off
for /F "tokens=1,* delims=-" %%a in ('dir /A-D /B "*.mp4"') do (
echo move "%%a-%%b" "%%a%%~xb"
)