For words that only have 3 letters, there's no 4th letter. One example is with the word “insert”, there aren't any a lot more letters afterwards and therefore should be the word "increase". The word “addict” is inside the BIP39 wordlist however , you would have made use of “addi” https://top10bookmark.com/story18824954/considerations-to-know-about-bip39-seed-phrase-generator