For words that have only 3 letters, there isn't a 4th letter. As an example with the word “include”, there aren't any extra letters afterwards and so has to be the word "add". The word “addict” is within the BIP39 wordlist but you might have utilized “addi” for “addict”. BIP39 https://gorillasocialwork.com/story20486173/facts-about-bip39-seed-phrase-github-revealed