[Google Pixel 4 XL Guides, News, & Discussion] relaxable:
As I posted in another thread earlier today, even though this Pixel 4XL is a huge disappointment in many ways to me, I still will consider buying it to replace my Nexus 6 - which is still doing decent job for me.

Google has never given us bright display, and this XDA article analyzing Pixel 4 displays explains it pretty well -

My question is - anybody with Pixel 4XL -> have you been able to get high-brightness mode for regular [while outdoors] display? Anyone can try?

To me, the state of pretty much every Google-branded phone and display is a step-behind the leaders. Here, we've got Pixel 4XL with a last-generation display that is dimmer and takes more power than Samsung's latest. And then, Google disables the high-brightness (high-battery-drain) mode - so you can't see the display in bright sunlight. So competitors using the same last-generation display have brighter phones (and better battery life). For me, if I can't see the damn display, it doesn't matter what the battery life is - the phone is unusable for lots of things if you can't see the display.

Recommend reading Dylan Raga's entire article, but here my highlight, and just so sad that Google is so bad in this area:

The Pixel devices have historically been unimpressive when it comes to display brightness. This year is no different. While every other major smartphone maker has made their OLEDs significantly brighter, Google has shown little-to-no progress. Google did manage to increase its newest phones’ brightness this year, from about 400 nits up to 450 nits, but it still leaves them as some of the dimmest flagship smartphones in recent years.

Lack of high brightness mode

The reason Google is so far behind is that they are refusing to incorporate a higher-power brightness state for their system brightness. Furthermore, Google is using last-generation display panels that cannot compete in power efficiency or in rated brightness with Samsung’s latest panels. What’s interesting is that Google has had a higher brightness mode within their phones, which they can tap into during HDR playback (or with root). But for reasons likely related to battery, Google does not allow their phones to use this extra brightness for normal use. Higher brightness modes do require significantly more power to drive — an 800-nit peak brightness state drains significantly more power than twice that of a 400-nit brightness state — but if the competition is able to support higher brightness levels and maintain better battery life than the Pixel devices, then Google is severely falling behind in both departments.

When enabling high brightness mode within the Pixel 4s’, their displays approach acceptable levels of brightness. At 600 nits, this ranks the Google Pixel 4 displays competitively with last years’ OLEDs in brightness. But in 2019, 600 nits is about the baseline for every major smartphone company, while the best are pushing 800 nits (100% APL). These are simply the limits of Google’s outdated panels, as the same panels found in the Huawei Mate 30 Pro and the OnePlus 7 Pro push the same brightness levels — except those phones actually push those brightness levels in normal use.​