For words that only have 3 letters, there is absolutely no 4th letter. For example with the word “incorporate”, there aren't any extra letters Later on and so need to be the word "add". The word “addict” is in the BIP39 wordlist however you would have employed “addi” for “addict”. https://getsocialnetwork.com/story4474274/facts-about-bip39-seed-phrase-github-revealed