Before updating to macOS 10.12.4, make a new installer on another flash drive and install using the flash drive. If you install w/o reformatting your drive, then you'll be able to keep all of your files.
I've had 2 people try to update and it has bricked their system. Once again, use another flash drive as an installer, and install without formatting your drive!
If you're having issues booting, change your SMBIOS to "iMac14,2" and see if it works. If not, just post a new discussion.