For words that only have three letters, there isn't a 4th letter. By way of example with the word “incorporate”, there are no extra letters afterwards and so have to be the word "increase". The word “addict” is in the BIP39 wordlist however, you might have made use of “addi” https://bookmarkworm.com/story18911967/facts-about-bip39-seed-phrase-github-revealed