For words that only have three letters, there is absolutely no 4th letter. One example is with the word “increase”, there aren't any more letters Later on and thus needs to be the word "incorporate". The word “addict” is while in the BIP39 wordlist but you might have used “addi” https://ztndz.com/story21895938/the-basic-principles-of-bip39-passphrase-generator