These Graphic settings are personal preference, it certainly doesn't look absolutely stunning in the end, but it's a good mix between power efficiency and good graphics. That said, you can customize your settings as different as you want. My settings make the game run for about 4 hours theoretically, but I didn't come to the point to test this, especially because I don't want to play this game for 4 hours straight through. You'll need to find that out yourself. I also did a few tests on an external monitor (Resolution 2560x1440) to test if it's still enjoyable on an external monitor. We'll get back to this later.

Also, in the beginning you will have lots of lagging gameplay, regardless of what settings you will use from this guide, because the game fills up it's shadercache while you're playing. It will be around 25GB big and you will have to drive around every single part of the map to have everything loaded. As soon as you have done that you will soon realize, that the game stops lagging.

At this point in time I use the default Compatibility settings, so it's probably running on a specific version of proton that it's downloading on the first startup of the game. I left the Game Resolution on Default, even on external Displays. The game will only run 720p Resolutions with these settings, also on external displays, because I want to focus on the smoothness on the gameplay and the graphics at the same time. The experience on external Displays will still not be completely fluent, because of some settings we'll be setting up later.

Now it's time to open up the Game so we can create a Performance profile for Forza Horizon 4. You can copy these Settings:
Refresh Rate: 40
Allow Tearing: On
Half Rate Shading: Off
Scaling Filter: Integer

Update: I found it to run more stable by locking the GPU Clock Frequency to 1100.

First disable dynamic optimization. We'll set some specific settings later.
I've seen a lot of people reducing the resolution so the game runs smoother, which doesn't help at all, so leave the resolution on 1280x800.
Set the Framerate to 60FPS (although we will never reach it).
Disable V-sync.
Don't activate full screen by any means!
Disable Motion Blur.

Now switch over to the advanced tab.
Apply all these settings:
Anisotrophic Filtering: Ultra
Night Shadows: On
Shadow Quality: Low (You won't recognize it when on handheld mode)
Environment Texture Quality: High
Static Geometry Quality: High
Dynamic Geometry Quality: Medium
MSAA: 4x (I found it to add a lot to the experience)
FXAA: Off
SSAO Quality: High
Reflection Quality: Medium
Windshield Reflection Quality: Medium
Mirror Quality: High
World Car Level of Detail: Medium
Deformable Terrain Quality: Medium
SSR Quality: Medium
Shader Quality: High

As I already said before, the game will stutter a bit on 60FPS, so if you're playing on an external monitor, the only way to have a smooth experience is to have a Display with variable Refresh rate. But still, I think it's quiet enjoyable on an external Display even with the stutters and the lower resolution.

If you have any other Recommendations, drop them in the comments, I may give them a shot. Anyways, have fun playing. If you want to have some new music in the game, I will shamelessly plug my Mod over on Nexusmods, that replaces the Radio of Forza Horizon 4 with the Music from Forza Horizon 3.[www.nexusmods.com] Have fun playing everyone.