Unfortunately there isn't any public change-log yet to reflect what's new with AMD Catalyst 13.4. Earlier Catalyst 13.3 Betas introduced Linux 3.8/3.9 support and Valve Source Engine fixes. The game fixes were namely for Team Fortress 2.
From my privileged AMD Linux information, it doesn't appear that there is much else to this Catalyst 13.4 Linux driver release besides more driver fixes.
The AMD Catalyst 13.4 for Linux Radeon / FirePro graphics driver release can be downloaded from AMD.com.
Update: AMD Catalyst 13.4 Linux release notes are available from this web-page.