Instead of searching for "part4" specifically, search for the full string "HV530 PC821 3840x2160" on the site where you found the first three parts.
If you find the file and it says "checksum error," it usually means the download was corrupted or you are using an outdated version of WinRAR/7-Zip. 3. Safety Warning Download HV530 PC821 3840x2160 part4 rar
If one part (like part4) is deleted from a file host due to inactivity, the entire set becomes unusable. You usually cannot mix and match "part4" from a different uploader unless the file hash is identical. Instead of searching for "part4" specifically, search for