The patch notes say that the “High Sierra 10.13.4 Update improves stability, performance, and security.” However, some users are finding that when they try to restart after installing they wind up in Recovery Mode with a message that says, “The macOS installation could not be completed.”
Diagnostics seem to show that disks are in good shape, so hardware malfunction can be ruled out. At least one user reported being able to get things running again by booting into Safe Mode, but another user reported on Apple Stack Exchange that his Mac stalled at 100 percent after booting to Safe Mode on Apple tech support's advice.
Cupertino does not appear to have any answers to what is causing the failures, but ExtremeTech and AppleInsider are also reporting stability issues for macOS 10.13.4 involving multi-monitor configurations.
“DisplayLink is used to allow computers to run displays via USB technology,” says ExtremeTech. “It’s deployed across a number of different products, and there are software solutions that leverage it in various ways, typically to extend or clone a Mac desktop. And as of macOS 10.13.4, none of those solutions seem to be working — or at least, they’re not working normally.”
“The macOS installation could not be completed.”
DisplayLink is not the only multi-monitor solution having problems. Duet posted an alert on its support pages warning users to hold off on updating to 10.13.4.
“We recommend staying on macOS 10.13.3 [emphasis Duet]. Installing macOS release Version 10.13.4 will cause Duet Display stop functioning after the OS upgrade [sic]. We have alerted Apple to this issue and are working hard to find a resolution. These features continue to work as expected in 10.13.3.”
It appears that at least in the Duet and DisplayLink cases users are able to start up normally but lose some display functionality. This is quite different from not being able to boot up at all.
In any case, it looks like Apple has some patch work to do to resolve multiple issues with the current version of High Sierra. For now, avoid updating to 10.13.4. If you feel you must install it, be sure you have a current backup on hand in case things go sour, which is a good policy for any major installation.