Many people use third party displays either in the form of a real monitor or an iPad attached to their Mac. Usually there are Apps that allow the display to connect to the Mac. In my case, I use Duet Display to connect my iPad as an extra monitor. Apparently, the High Sierra 10.13.4 update breaks these connector Apps. The Developers are working on their Apps. So, if you run one of these Apps, you might check with the Developer to see if it works before installing 10.13.4.