Be sure to back up any important data, and if possible clone your hard drive before applying the update. If you have any issues post-update, boot using UniBeast and type UseKernelCache=No.
What we know about 10.9.4 so far...
- OS X Build 13E28
- 13.3.0 Darwin kernel
- Updated Intel, NVIDIA, and AMD graphics drivers
- If you use patched or modified drivers, you will need to re-apply changes to the updated drivers.
- NVIDIA Alternate Graphics Drivers not updated yet for 10.9.4.
- This update will replace your audio drivers.
- You will have to reinstall AppleHDA using MultiBeast before or after reboot
- If you use VoodooHDA, you will not need to make any modifications
- Storage drivers updated - SSD TRIM patch will not work with 10.9.3 patch. Fix will be available in the next MultiBeast.
- Install OS X Mavericks App Updated - If you downloaded Mavericks from the App Store prior to today, and you want to have the latest version on your backup installer USB, simply log into the Mac App Store and click Download. When the download finishes, the app will automatically open. The updated 10.9.4 Install OS X Mavericks App can be found in the Applications folder. You may then re-create your UniBeast installation USB.
Source: http://www.tonymacx86.com/438-os-x-10-9-4-update.html