For words that have only 3 letters, there is no 4th letter. By way of example with the word “incorporate”, there aren't any extra letters Later on and therefore has to be the word "insert". The word “addict” is within the BIP39 wordlist however you would've made use of “addi” https://worldsocialindex.com/story4526657/a-review-of-bip39-phrase-list