Since Techical Communication Suite 4 has such a large footprint for installation, I need to free up some disk space. Can I uninstall TCS3 before upgrading to TCS4?
You can, but remember that TCS 4 doesn't include Photoshop - you get Illustrator instead, so if you need PS, either don't remove TCS3 or uninstall it & reinstall TCS3 but only select PS for installation.
Different versions of TCS can co-exists independently and TCS supports custom install/uninstall. So you can unsintall all the products that you do not require from TCS3 and install whatever needed from TCS4.