For words that only have three letters, there isn't any 4th letter. By way of example with the word “add”, there aren't any additional letters afterwards and thus have to be the word "add". The word “addict” is while in the BIP39 wordlist however, you would've applied “addi” for “addict”. https://onlybookmarkings.com/story18754608/detailed-notes-on-copyright-wallet-seed-phrase-out-of-bip39