For words that only have three letters, there is absolutely no 4th letter. Such as with the word “add”, there aren't any more letters afterwards and therefore have to be the word "insert". The word “addict” is within the BIP39 wordlist however , you would've utilised “addi” for “addict”. You https://maximusbookmarks.com/story19001010/facts-about-bip39-seed-phrase-word-list-revealed