For words that only have 3 letters, there isn't a 4th letter. By way of example with the word “include”, there are no more letters Later on and thus needs to be the word "incorporate". The word “addict” is within the BIP39 wordlist however you would've employed “addi” for “addict”. https://minibookmarking.com/story19019857/the-ultimate-guide-to-bip39-seed-phrase-mnemonics-generator