[kbart]

It's vulnerable to the dictionary-based attacks that are very common.

[willis936]

Diceware is designed to make passwords against dictionary attacks. Estimates of diceware entropy begin with the assumption that an attacker has the dictionary. A dictionary with 6^5 entries would take 6^5^N guesses to exhaust (assuming the entries are randomly chosen). 6^5^4 = 2^52.

[rini17]

That is a sadly too often repeated lie. If you know otherwise please explain/link how the attack works, how can you guess the 4 words? Effectively, that would mean requiring much less than 2^44 attempts as xkcd explains.