To actually use this file, you cannot open it in isolation. You generally need:

You right-click Part 1 only and select "Extract Here." The software will automatically pull data from Part 4 and the others to recreate the original game folder. Safety & Source Note

Are you having trouble this specific part, or2.1?

The .part4.rar extension means this is the fourth piece of a larger "split" archive. Because high-quality game files are often several gigabytes, uploaders split them into smaller chunks (Part 1, Part 2, etc.) for easier hosting and downloading. How to use it

You must have every part of the set (e.g., Part 1 through Part 5) in the same folder.

Software like WinRAR or 7-Zip is required to recombine them.