For words that have only 3 letters, there is absolutely no 4th letter. For instance with the word “incorporate”, there aren't any more letters Later on and therefore need to be the word "add". The word “addict” is while in the BIP39 wordlist but you would've utilised “addi” for “addict”. https://bookmarkoffire.com/story18731271/helping-the-others-realize-the-advantages-of-bip39-mnemonic-phrase-tool