For words that only have 3 letters, there is not any 4th letter. Such as with the word “add”, there won't be any much more letters Later on and so should be the word "insert". The word “addict” is from the BIP39 wordlist however you might have made use of https://bookmarkinglog.com/story18949648/considerations-to-know-about-bip39-seed-phrase-generator