For words that have only 3 letters, there isn't any 4th letter. One example is with the word “insert”, there aren't any additional letters afterwards and so need to be the word "incorporate". The word “addict” is within the BIP39 wordlist however , you would have applied “addi” for “addict”. https://ragingbookmarks.com/story18805931/little-known-facts-about-bip39-seed-phrase-list