Multi-volume (Split) Archive. The ".part04" suffix indicates it is not a standalone file but a piece of a sequence.

You cannot open this segment individually; it contains only a fraction of the total data.

Newer RAR naming conventions use the .partXX.rar pattern.

Split RAR files - what they look like and how they work with WinZip

RAR (Roshal Archive), a proprietary compression format.

RAR archives provide high-quality compression and can include features like AES-256 encryption or recovery records to repair damaged data. How to Use This File