For words that only have 3 letters, there isn't any 4th letter. As an example with the word “add”, there isn't any additional letters afterwards and so should be the word "insert". The word “addict” is during the BIP39 wordlist however, you would have applied “addi” for “addict”. BIP39 and https://socialdummies.com/story3674902/bip39-seed-phrase-fundamentals-explained