Fix all your shadowing problems by doing the following:
Nvidia / AMD users FXAA and anti-aliasing must be completely turned off.
Make the following modifications to your Fallout4Prefs.ini:
sAntiAliasing=TAA
fLeafAnimDampenDistEnd=50.0000
fLeafAnimDampenDistStart=0.0000
iShadowMapResolution=3000
fDirShadowDistance=1500.0000
fShadowDistance=1500.0000
iVolumetricLightingQuality=0
bVolumetricLightingEnable=0

These are the best settings that the game can use without visually glitching in some way.
If you want higher quality shadows increase iShadowMapResolution.
If you want shadows at greater distances increase fDirShadowDistance, and make fShadowDistance the same.
Save.

Make the following modifications to your Fallout4.ini:
bNvGodraysEnable=0
iShadowMapResolutionPrimary=3000
fSunUpdateThreshold:Display=20
fSunShadowUpdateTime=0.02
Save.

Make both files read-only.
All shadow problems should now be solved.