Installation of 10.13.4 went well on my MacBook Pro (Retina, 15-inch, Mid 2014). It took 20-30 minutes. As many have mentioned, expect several reboots and a lot of time with nothing but a black screen showing. If you see a black screen, wait at least 30 minutes before try to tamper with it because it is possibly/probably still working on the update. Apple really needs to figure out a method to keep some indicator on the screen or light on the Mac so we know the update is still running. I suspect a lot of the issues we are reading about here in the forum are because the screen went black and the Mac user panicked and tried to restart the Mac manually. Again, the best solution if the Mac screen goes black is to wait at least 30 minutes to see whether it is still running.
Rule of thumb for Mac Updates: Black screen during update is probably okay and update is likely still running so don't touch the Mac for at least 30 minutes if the screen goes black. Be PATIENT!!!