For words that only have three letters, there isn't any 4th letter. For instance with the word “insert”, there won't be any much more letters afterwards and thus need to be the word "insert". The word “addict” is within the BIP39 wordlist but you would have applied “addi” for “addict”. https://orangebookmarks.com/story18906963/bip39-phrase-generator-can-be-fun-for-anyone