For words that only have 3 letters, there isn't any 4th letter. For example with the word “increase”, there aren't any more letters afterwards and thus has to be the word "include". The word “addict” is in the BIP39 wordlist however, you would've applied “addi” for “addict”. Yet another advantage https://advicebookmarks.com/story26402202/bip39-phrase-generator-can-be-fun-for-anyone