Since you are looking for Part 4 specifically, you likely already have some parts. To avoid corruption, ensure:
Move all files into one dedicated folder on your computer.
To download and use the file, you must ensure you have all other associated parts (Part 1, Part 2, etc.) in the same folder before extracting them together to obtain the single firmware image. 1. Download All Parts
Every part must come from the same source to ensure they are versions of the same split archive.