For words that only have 3 letters, there's no 4th letter. One example is with the word “add”, there isn't any extra letters afterwards and so have to be the word "add". The word “addict” is in the BIP39 wordlist however, you would've used “addi” for “addict”. bip39-phrase.com is definitely https://bookmarkfox.com/story4334904/a-review-of-bip39-phrase-list