For words that only have 3 letters, there is absolutely no 4th letter. For example with the word “add”, there aren't any a lot more letters afterwards and therefore has to be the word "add". The word “addict” is from the BIP39 wordlist however you would've employed “addi” for “addict”. https://redhotbookmarks.com/story18760549/the-invalid-bip39-phrase-diaries