You cannot open part4 in isolation. You must have every preceding part (part1 through part3) and any subsequent parts in the same folder.
The suffix indicates that this is a "split archive." Large video files are often compressed and divided into multiple segments (Part 1, Part 2, etc.) to make them easier to upload and download on file-sharing sites.
Many websites that claim to host these specific file parts are designed to steal user data or force clicks on malicious advertisements (malvertising).