For words that have only 3 letters, there is no 4th letter. As an example with the word “add”, there won't be any far more letters Later on and therefore have to be the word "include". The word “addict” is during the BIP39 wordlist however you would've made use of https://guidemysocial.com/story4200881/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-definition