For words that only have 3 letters, there isn't any 4th letter. For instance with the word “add”, there won't be any additional letters afterwards and so need to be the word "add". The word “addict” is in the BIP39 wordlist however, you might have utilised “addi” for “addict”. You https://socialwoot.com/story20768109/not-known-facts-about-bip39-english-words