For words that have only 3 letters, there's no 4th letter. As an example with the word “insert”, there are no much more letters afterwards and thus should be the word "insert". The word “addict” is within the BIP39 wordlist but you would have employed “addi” for “addict”. Mnemonic words https://geilebookmarks.com/story18741842/phrases-with-bit-an-overview