For words that have only 3 letters, there's no 4th letter. One example is with the word “include”, there isn't any additional letters Later on and so need to be the word "include". The word “addict” is in the BIP39 wordlist however you would have used “addi” for “addict”. Mnemonic https://bookmarkchamp.com/story19413619/bip39-seed-phrase-mnemonics-generator-an-overview