If your Mac has been freezing up since you applied the OS X 10.11.4 update to El Capitan, then take comfort in the fact that you're not alone.
There are widespread reports of systems freezing up following the updates. The majority of these freezing system - although not all - appear to be 13-inch Retina MacBook Pros (Early 2015) and 15-inch MacBook Pro (Mid 2015).
The freeze-ups are also being reported by users who have installed the Safari Technology Preview Version 1 and OS X 10.11.5 Public Beta 1.
Here's a video of what happens:
The freeze-ups appear to be random, and they require the system to be hard rebooted to bring it back to life. Unfortunately, this is only a short-term fix, and some users have been forced to either rollback their systems to 10.11.3 using a Time Machine backup, or reinstalling OS X from scratch (both of which are time-consuming activities).
If you haven't yet updated to OS X 10.11.4, then you might want to hold off upgrading until this issue is addressed.