For words that only have three letters, there is no 4th letter. For example with the word “insert”, there aren't any more letters afterwards and thus needs to be the word "incorporate". The word “addict” is within the BIP39 wordlist however, you might have used “addi” for “addict”. A seed https://socialmphl.com/story20765555/the-definitive-guide-to-bip39-seed-phrase