For words that only have 3 letters, there is no 4th letter. By way of example with the word “incorporate”, there aren't any additional letters Later on and so need to be the word "increase". The word “addict” is from the BIP39 wordlist however, you would've used “addi” for “addict”. https://bookmarkprobe.com/story19250855/the-definitive-guide-to-bip39-seed-phrase