For words that only have three letters, there isn't any 4th letter. One example is with the word “increase”, there are no much more letters afterwards and thus has to be the word "add". The word “addict” is from the BIP39 wordlist however, you would have applied “addi” for “addict”. https://socialrator.com/story9464297/the-ultimate-guide-to-bip39-seed-phrase-github