For words that only have 3 letters, there is absolutely no 4th letter. One example is with the word “add”, there are no more letters Later on and thus need to be the word "increase". The word “addict” is from the BIP39 wordlist but you would've utilised “addi” for “addict”. https://dftsocial.com/story19598237/examine-this-report-on-bip39-passphrase