For words that only have three letters, there's no 4th letter. For instance with the word “increase”, there aren't any extra letters afterwards and so has to be the word "add". The word “addict” is from the BIP39 wordlist however you would have utilised “addi” for “addict”. In this way, https://ragingbookmarks.com/story18906144/little-known-facts-about-bip39-words