For words that have only three letters, there isn't a 4th letter. For example with the word “add”, there are no extra letters afterwards and thus should be the word "add". The word “addict” is inside the BIP39 wordlist but you might have made use of “addi” for “addict”. Safety: https://ez-bookmarking.com/story18778226/considerations-to-know-about-bip39-seed-phrase-generator