Yes, you can overwrite all file just unpacked the file of stable package or you can find the different between RC4 package and stable package with a comparison software (likes meld for linux and winmerge for windows) and overwrite only the different files.
In the upgrading tutorial you can read that there isn't difference in database between RC4 and stable version. If you do this your site will upgrade to stable version. Remember make a backup before.

I updated my Joomla! 1.5RC4 with stable package and now it works very well.
I overwrite all file.
I verified that there wasn't difference in databases between 1.5RC4 and 1.5stable (see installation/sql/mysql/diff.sql).