For words that have only 3 letters, there's no 4th letter. For instance with the word “increase”, there aren't any more letters afterwards and therefore has to be the word "increase". The word “addict” is while in the BIP39 wordlist however you would have applied “addi” for “addict”. You should https://bookmarkmoz.com/story18866827/little-known-facts-about-bip39-seed-phrase-list