Where can I download camera raw 7.3 update for CS4 to read CR2 files from Canon 6d?
Camera Raw version 7.x, including 7.3 of course, does NOT run in CS4.
Forget about that, you will never be able to open raw files from that camera directly in CS4, as it has been discontinued for years and there will be no further updates to it.
Your choices are to upgrade to at least CS6, or to download the free, stand-alone Adobe DNG Converter 8.1, install it, launch it and convert your CR2 raw files to raw DNG files that you can then open in CS4.
Note that the DNG Converter works only on the folders that contain the raw files. It will not see the individual files contained therein.