If you own the game on , you can right-click RimWorld > Properties > Betas and select version-1.4.3901 from the dropdown to get a complete, verified copy.
Look in your Windows Defender or antivirus history. RimWorld.v1.4.3580.zip
It looks like you are referencing a specific archived version of (v1.4.3580). If you are looking for a "piece" in the context of a missing file, a mod, or a specific fix for that version, I can point you in the right direction depending on what you need. If you own the game on , you