For words that only have 3 letters, there isn't any 4th letter. For example with the word “insert”, there isn't any much more letters afterwards and so need to be the word "incorporate". The word “addict” is within the BIP39 wordlist but you would have used “addi” for “addict”. Mnemonic https://hubwebsites.com/story20828286/the-greatest-guide-to-seed-phrase-bip39