For words that only have three letters, there's no 4th letter. For example with the word “increase”, there aren't any extra letters afterwards and so have to be the word "include". The word “addict” is in the BIP39 wordlist however , you would have applied “addi” for “addict”. In this https://scrapbookmarket.com/story18807937/the-ultimate-guide-to-bip39-seed-phrase-github