Url rewrite and forced 404

We serve our content via ourdomain.com/folder.

The next iteration of the application is ready to be rolled out in ourdomain.com/folder2.

If a user tried to access ourdomain.com/folder2 directly he would get a 404.

I need a to get a rewrite rule that serves the content of ourdomain.com/folder2 under the url ourdomain.com/folder and gives the 404 for direct access to ourdomain.com/folder2.

Why bother with mod_rewrite? Can't you just rename the subdirs?

mv folder folder.old
mv folder2 folder