For words that only have 3 letters, there's no 4th letter. For instance with the word “add”, there are no much more letters afterwards and so has to be the word "add". The word “addict” is while in the BIP39 wordlist however , you would've utilised “addi” for “addict”. This https://yourbookmarklist.com/story19015120/a-review-of-bip39-phrase-list