For words that have only 3 letters, there isn't any 4th letter. For example with the word “insert”, there are no additional letters afterwards and so has to be the word "add". The word “addict” is while in the BIP39 wordlist however you would've utilized “addi” for “addict”. Art is https://thebookmarkplaza.com/story18800816/the-greatest-guide-to-seed-phrase-bip39