Using a 10.9.2 USB UniBeast install drive, I couldn't boot to get my 10.9.3 update to work for the same issue with kernel panic; so, I ended up reinstalling it with a newly created 10.9.3 USB UniBeast install and had no problem. Now, with 10.9.4, I faced the same issue with updating from a 10.9.3 install until I figured out the problem--something to do with my flash drive. I think it is a USB issue since the only way I could get to the desktop so I could run the ProBook installer without completely reinstalling was by removing the USB drive before the system started loading the desktop.
I noticed that just before the desktop tried to load, there was some kind of final communication with the USB drive and then no further loading took place. The drive would always stop flashing just before the desktop would freeze. It appears that there must be an issue with some file(s) preventing the desktop loading since the same drive loads the desktop if I do a fresh install. Could there be some conflict between the install of an older installer and the new updates? Remember, in both cases of upgrading, I was running the previous build on my USB drive while booting to run the ProBook installer.