For words that only have three letters, there is no 4th letter. As an example with the word “insert”, there won't be any additional letters Later on and thus needs to be the word "incorporate". The word “addict” is while in the BIP39 wordlist however, you would've made use of https://getsocialsource.com/story4310758/not-known-factual-statements-about-bip39-explained