For words that only have three letters, there's no 4th letter. As an example with the word “insert”, there won't be any additional letters Later on and so has to be the word "increase". The word “addict” is while in the BIP39 wordlist however you would have applied “addi” for https://bookmarketmaven.com/story19323148/facts-about-bip39-seed-phrase-github-revealed