Displaying on external screen stopped working after todays MacOS Update

Safe Boot from the HD, (holding Shift key down at startup), does the problem occur in Safe Mode?

Safe mode attempts to repair Disks & clears lots of caches, so if Safe Mode works try again in regular boot.

Have you done a PRAM reset, CMD+Option+p+r...

http://support.apple.com/kb/HT1379

In fact, do 3 in a row, takes a bit of time.

Intel-based Macs: Resetting the System Management Controller (SMC)...

http://support.apple.com/kb/HT3964

Also experiencing a similar issue after upgrading to 10.15.4.

Running a 13-inch MBP with touchbar, Dell 4K monitor (U3219Q) connected via USB-C to USB-C. I'm experiencing intermittent issues with the external monitor: poor quality display (only running with a 2K resolution @ 24 Hz) or simply not displaying anything, gray/black, then showing at a 2K resolution. (It normally runs at a 4K resolution @ 60 Hz.)

Sometimes, I'm plugging it in and getting a 4K @ 60Hz quality picture, other times it'll only run at a 2K resolution. Doesn't matter which USB-C port I plug it into. Also, sometimes System Preferences will decide to allow me to select options for the monitor, sometimes the options will be grayed out.

I'm also getting some weird things in the Displays options in System Preferences; normally, the MBP picks up and shows DELL U3219Q but I've seen some weirdness like "Unknown Monitor Type" ("Moniteur de type inconnu") or even macOS referring to the external monitor as a "Projector" ("Projecteur"), which it is definitely not.

I've tried resetting the SMC, PRAM/NVRAM but doesn't seem to help. Apple Support's only suggestion is to perform a clean install, which I could do (have full backups), but it's an incredibly inconvenient suggestion and I wish to avoid doing it.

Did not have this issue prior to 10.15.4. Also, I have tested on a second 4K Dell monitor (and with different cables)—getting the same results. An older MacBook I have running 10.15.3 is not having any issues at all.

For me those options didn't work. Reset of SMC and PRAM made the external display work until the moment I closed the lid and tried to use the MBP in clamshell mode. After that, the issue repeated itself. Clamshell mode stayed broken. Option of last resort was roll-back to 10.15.3.

Take away for Apple: integrate the roll back to previous version.

Another thing I discovered in a last attempt before rolling back, this MBP: try another USB-C port. This MBP 16" has 4 USB-C connections. While normally the top right USB port was the port for output, that has reversed after 10.15.4. Connecting the monitor to the top-right still crashes the driver: screens (internal and external) blink: black, dark grey, black, dark grey... rinse repeat. All of the sudden: the lower-left USB-C port works and my external monitor (for now) gives a screen.

This seriously is a driver issue.

Great news, thanks for the report! :)

I have the exact problem. Made sure monitor and HDMI cable works by connecting to a Windows machine and it works. So the problem is due to the upgrade.