For words that have only three letters, there isn't a 4th letter. As an example with the word “add”, there isn't any a lot more letters afterwards and thus must be the word "add". The word “addict” is within the BIP39 wordlist but you would have used “addi” for “addict”. https://altbookmark.com/story20428428/a-review-of-bip39-phrase-list