Cisco created Type 4 around 2013 in an attempt to strengthen password, unfortunately the attempt was severely flawed and resulted in a hash that was weaker than a Type 5 MD5. See the PSIRT below.
Cisco IOS and Cisco IOS XE Type 4 Passwords Issue
These use a very simple MD5 hashing algorithm. These are easily reversible with tools on the internet. These should only be used if Type 9 is not available on the IOS version you are running.
These use the Vigenere cipher, a very simple algorithm that was cracked in 1995. These are easily reversible with tools on the internet. These should never be used.
Type 8 passwords are what Type 4 was meant to be, PBKDF2, SHA-256, 80 bit salt, 20,000 iterations. While this is good, it is still vulnerable to brute forcing since AES is easy to implement in graphics cards. I have not proven it but I believe it is possible that the popular tool HashCat is able to decrypt these.
These use the SCRYPT hashing algorithm SCRYPT, 80 bit salt, 16384 iterations. It’s expensive to run the algorithm and therefore currently the Best Practice Type password to use. I have not proven it but I believe it is possible that the popular tool HashCat is able to decrypt these.
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