Just because I’m stubborn, I took a chance and tried again with the software update. As before, the install froze up and failed. This time, I managed to repeat the steps to get out, shut down, and power up, and fortunately I was able to get back in. I’ve learned my lesson—they can keep their dysfunctional El Capitan 10.11.4! I’m sure I can continue to manage without it. I’m just happy to be able to post this. If anyone out there is familiar with this glitch, please feel free to advise.