On https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ne-d3d12-d3d12_tiled_resources_tier are four possible tiers for Tiled Resources. But the fourth tier is undocumented in contrast to the others, so I can't figure out what this tier is for. What additional features does tier 4 offer? Which GPUs support tier 4? And since which Shader Model/Windows 10/WDDM version is this tier available?
The D3D12_TILED_RESOURCES_TIER4 enum was added for Windows 10 SDK (17134) as an experimental value, but to date it's never been implemented nor does any current hardware support it.
You can also look at the public specs, but this particular question isn't addressed in those docs.