When i change server side less compilation on client side, i have 404 with all my less files.
Client side compilation only works in developer mode. So just in case, please make sure you are in developer mode.
After you do that, and you only need to do this once:
rm -R pub/static/frontend
rm -R var/view_preprocessed
php bin/magento cache:flush
After that, refresh the page and you should see your changes. You should be able to see subsequent changes with a simple page refresh and you shouldn't have to repeat the other steps above.