Whoops — my bad! The option appears grayed out if (like me) you have some users on machines which have PHP 5.4 available, but the currently selected user isn't on them. Chances are that none of you are on those machines (mostly because the only one that exists is one of my test VPSes), so you aren't seeing that part of the UI yet.
We're in the process of rolling out the PHP 5.4 package. Once that's done, we'll flip the switch to mark it as "available" on your machines, and you can proceed to use it.