You cannot open "part4" alone. Ensure you have parts 1, 2, and 3 downloaded. Open part1 with a tool like WinRAR or 7-Zip, and it will automatically pull data from the other parts to recreate the full file.
.part4 (this is the 4th segment of a multi-part download; you need all parts—e.g., part1, part2, etc.—in the same folder to extract the contents). Key Features of Version 1.8.2.0 sc24857-FS22v1820.part4.rar
Essential for running newer mods that require the 1.8.2.0 framework. Usage Notes You cannot open "part4" alone