For words that only have three letters, there is absolutely no 4th letter. As an example with the word “add”, there aren't any more letters afterwards and so need to be the word "incorporate". The word “addict” is during the BIP39 wordlist however you would've utilised “addi” for “addict”. You https://bookmarkbells.com/story18897144/getting-my-bip39-words-to-numbers-to-work