As you may have seen a few hours ago, Developer Preview 4 of OnePlus 8-series got released.
Firstly, gaming tools box - mentioned in Game Space part of the changelog - will allow you to easily trigger Fnatic Mode, notification block, mistouch prevention, screen recorder and eventually monitor data being used as well. You should also be able to see in real-time temperature of your device as well as its battery level.
Below are the drawables extracted from Game Space APK.
Landscape mode drawable
Portrait mode drawable
Secondly, updated Camera application (5.4.74) hints at 8K video support for upcoming devices, most likely the OnePlus 8T, since it may have a 64 MP sensor - which I spotted in version 5.4.23 - extracted from Developer Preview 2 and tipped to XDA-Developers.
It also hints that future devices (maybe "billie" family, who knows?) may have an SD-card slot, which would be a precedent as only the OnePlus X previously had one. Below are strings and shared preferences demonstrating above.
On top of that, Settings APK informs us of two small new features: hiding silent notifications in status bar and a new accent colour. It also includes an image of the OnePlus 8T which will be used in About Phone section.
Slightly less important but nonetheless interesting, Messages APK got bumped from 220.127.116.11.200808005701 to 18.104.22.168.200828005702 and adds a few strings related to Rich Communication Services, which means it may be supported in the future.