186.rar

Open it with a hex editor; RAR4 starts with 52 61 72 21 1A 07 00 , while RAR5 starts with 52 61 72 21 1A 07 01 00 .

To give you the most accurate solution, could you tell me (e.g., a specific CTF like Hack The Box or PicoCTF) or what happens when you try to open it ? 186.rar

If the file won't open, the magic bytes or block headers might be intentionally damaged. Open it with a hex editor; RAR4 starts