This is as intended. We do not officially support the editor on SM4.
Attempting to run the UE4 Editor using SM4 causes an error to popup stating that "A Feature Level 5 video card is required to run to Editor. AvailableFeatureLevel = SM4, ShaderPlatform = PCD3D_SM4"
This was initially reported on Linux but it couldn't be reproduced. This is reproducible on Windows using the repro steps. Licensee was able to workaround the issue by removing the lines that were added to Engine/Source/Runtime/RHI/Private/DynamicRHI.cpp. (https://github.com/EpicGames/UnrealEngine/commit/ede472db1761e155ed7b77b3af6b22b2f83dd267#diff-07bb5f15ad03c9abf07343e376474a80R186)
This is issue occurs in 4.20.3 (CL-4369336), 4.21 (CL-4541578), and Main 4.22 (CL-4569646)
Results: Error pops up stating that "A Feature Level 5 video card is required to run to Editor."
Expected: The Editor loads up without issue
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-66641 in the post.