The Raspberry Pi 4 can drive up to two displays, with a resolution up to 1080p at a 60Hz refresh rate. At 4K resolution, if you connect two displays then you are limited to a 30Hz refresh rate. You can also drive a single display at 4K with a 60Hz refresh rate: this requires that the display is attached to the HDMI port adjacent to the USB-C power input (labelled HDMI0). You must also enable 4Kp60 output by setting the hdmi_enable_4kp60=1 flag in config.txt.
$ sudo rpi-eeprom-update *** UPDATE AVAILABLE *** BOOTLOADER: update available CURRENT: Thu 3 Sep 12:11:43 UTC 2020 (1599135103) LATEST: Thu 29 Apr 16:11:25 UTC 2021 (1619712685) RELEASE: default (/lib/firmware/raspberrypi/bootloader/default) Use raspi-config to change the release.
VL805_FW: Dedicated VL805 EEPROM VL805: up to date CURRENT: 000138a1 LATEST: 000138a1