For words that only have three letters, there is no 4th letter. For example with the word “add”, there aren't any extra letters afterwards and therefore must be the word "increase". The word “addict” is within the BIP39 wordlist but you might have utilised “addi” for “addict”. In this article https://bookmarkeasier.com/story18689922/the-fact-about-valid-bip39-seed-phrase-that-no-one-is-suggesting