For words that have only 3 letters, there isn't any 4th letter. For example with the word “insert”, there aren't any far more letters afterwards and thus needs to be the word "insert". The word “addict” is while in the BIP39 wordlist however , you would have made use of https://ticketsbookmarks.com/story18748499/invalid-bip39-phrase-things-to-know-before-you-buy