For words that have only three letters, there is absolutely no 4th letter. For example with the word “incorporate”, there won't be any more letters Later on and therefore have to be the word "increase". The word “addict” is during the BIP39 wordlist however, you would've made use of “addi” https://wiishlist.com/story19646177/a-secret-weapon-for-bip39-words-to-numbers