For words that only have 3 letters, there is no 4th letter. By way of example with the word “add”, there isn't any a lot more letters afterwards and so should be the word "incorporate". The word “addict” is during the BIP39 wordlist however you would've utilized “addi” for “addict”. https://cyberbookmarking.com/story19400709/bip39-seed-phrase-fundamentals-explained