The bad news is that you will need to have root access to do it. High Brightness mode allows a bump in peak luminance for the screen of the Pixel 4 devices from about 450 nits to about 610 nits. If the brightness of the screen makes it hard for you to read the screen outdoors, this big bump in brightness might fix that issue for you.
Another potential caveat is that the only time the Pixel 4 can use High Brightness Mode is when playing HDR video. While the mode isn't an option in any user-accessible menu, Google did leave access to High Brightness Mode in the kernel. With root access achieved, the shell command to enable the mode is:
echo on >> /sys/class/backlight/panel0-backlight/hbm_mode
According to XDA-Developers High Brightness Mode appears to kick in when the screen is at maximum UI brightness levels. The only downside to enabling the mode is a reduction in battery life, which is likely the reason Google left the mode disable on the device by default. Another caveat is that the mode turns itself off whenever the screen is turned off.
However, enabling it can be automated with an app like Tasker. Those on the fence about buying a Pixel 4 XL can check out our review of the smartphone, where we did talk about screen not being up to par with flagship devices like the Samsung Galaxy S10.