For words that have only 3 letters, there is absolutely no 4th letter. As an example with the word “include”, there isn't any additional letters afterwards and so needs to be the word "incorporate". The word “addict” is inside the BIP39 wordlist but you might have employed “addi” for “addict”. https://fatallisto.com/story8563608/considerations-to-know-about-bip39-seed-phrase-generator