For words that only have three letters, there's no 4th letter. By way of example with the word “add”, there isn't any much more letters afterwards and therefore must be the word "insert". The word “addict” is in the BIP39 wordlist however , you would've employed “addi” for “addict”. Artwork https://getidealist.com/story20539400/not-known-facts-about-bip39-english-words