For words that have only three letters, there is absolutely no 4th letter. By way of example with the word “add”, there isn't any a lot more letters Later on and therefore should be the word "incorporate". The word “addict” is within the BIP39 wordlist but you would've utilised “addi” https://icelisting.com/story20018842/getting-my-bip39-words-to-numbers-to-work