How to solve some strange hash of hack.chat

[DanielG]

This is not something hashcat can do. Also the first 6 bytes of base64 only gives you the first 4 bytes of the hash. So you lose 224 bits of info about the hash. This will result in *a lot* of false positives for the password+SALT combination resulting in those 4 bytes.

I doubt this is worth your time.