"Part 4" is functionally useless on its own. To extract the game, you must have all preceding and succeeding parts (Part 1, Part 2, Part 3, etc.) in the same folder.
Large games like Fallout 4 (which can exceed 30–50 GB with all DLCs) are often split into smaller "parts" for easier uploading and downloading.
If you are looking at this specific version (1.10.50) for purposes: