For words that have only three letters, there is no 4th letter. By way of example with the word “add”, there isn't any extra letters Later on and therefore must be the word "insert". The word “addict” is during the BIP39 wordlist but you would've made use of “addi” for https://rankuppages.com/story4058572/the-definitive-guide-to-bip39-seed-phrase