For words that have only three letters, there is absolutely no 4th letter. By way of example with the word “insert”, there aren't any much more letters Later on and so should be the word "incorporate". The word “addict” is inside the BIP39 wordlist however , you would've utilised “addi” https://mediajx.com/story21470775/new-step-by-step-map-for-bip39-english-words