It's very easy to run a dual monitor setup with the new Raspberry Pi 4 Model B and it's two micro HDMI slots. (Attention, you need to use a HDMI cable with a micro HDMI plug).
Here are the steps on how to make a dual-display setup for Raspberry Pi 4.
Step 1, Insert the SD card
Step 2, Connect your left monitor
Step 3, Connect your right monitor
Step 4, connect your mouse and keyboard.
Step 5, power your raspberry pi 4
The best feature of the new Raspberry Pi 4 is its true 4K 60HZ output. The new board is capable of decoding 4K60 - video at 4K resolution and 60 frames per second with HEVC - in hardware.
You can also run your Raspberry Pi 4 at 4K 60HZ monitor. It's possible to watch 4K youtube videos with this mini pc + 4k portable monitor combo. But please make sure that the fan is cooling down the board. Otherwise, the board will overheat quickly.
If you're interested in dual vertical 4K monitors setup, you can join this topic on the Raspberry Pi forum.
A bit of backstory may be helpful, this is a digital menu for a local bar running on a pair of 4k Samsung TVs, with a chromium instance running on each monitor. The panels are set up vertically, and while 1920x1080 works it leaves a black border around the image, and the panels helpfully don't have a way to change the resolution to 1920x1080. Both displays are connected via HDMI with no converters or anything in between. The model is raspberry pi 4 with 4GB of RAM.
After getting it working I wiped and reinstalled with a fresh noobs image to remove stuff I put on their while getting it figured out. I ran all updates and only changes are installing unclutter and adding the commands to lxsession to open the chromium instances on each monitor and navigate to a webpage.
I'm going to work on it this evening so I'll include some screenshots of what i'm seeing.