Almost everything is set to ultra as you can see according to Digital Foundry analysis.
Here you can see results from PC version. Both RX 580 and GTX 1060 can run this game at 4K 30fps, but if you want locked 60fps then you need 1080ti
Forza Horizon 4 Game at High Quality setting benchmarks comparing multiple gpus performance in frames per second at different settings. Can you run it? check out the benchmarks!
www.gpucheck.com
Even if you will set everything to "high" RX 580 is still not fast enough to run this game at 4K 60fps (47fps average)
When it comes to Forza 7 here's GTX 1060 running ultra settings at 4K
34fps dips, so I dont think game can run at ultra settings on 1060 (at least not at 4K). What's more even at high settings game still dips below to 60fps (to around 52fps)
To sum it up people need a decent PC just to match xbox x settings and high end PC to play these games with maxed out settings at 4K 60fps. If xbox x can already run many games at 4K native then I'm not worried about next gen consoles. XSX should offer around 3-4 better framerate in the same game compared to xbox x. 12TF Navi is comparable to 15 GCN Vega, and VRS can boost performance up to 75% (according to 3dmark results). PS5 even at 9TF also should run many games at 4K 60fps.... although I still hope PS5 will end up with 12TF GPU as well

(for now I trust insiders like Osiris or Klee).