For words that only have three letters, there isn't a 4th letter. As an example with the word “include”, there aren't any extra letters afterwards and thus have to be the word "incorporate". The word “addict” is in the BIP39 wordlist but you would have applied “addi” for “addict”. You https://e-bookmarks.com/story4771084/fascination-about-bip39-english-words