For words that only have 3 letters, there is absolutely no 4th letter. One example is with the word “include”, there are no more letters Later on and so must be the word "incorporate". The word “addict” is inside the BIP39 wordlist but you might have employed “addi” for “addict”. https://bookmarksystem.com/story19305233/the-ultimate-guide-to-bip39-seed-phrase-github