For words that only have three letters, there is not any 4th letter. One example is with the word “incorporate”, there isn't any additional letters afterwards and therefore have to be the word "incorporate". The word “addict” is while in the BIP39 wordlist however, you would've made use of “addi” https://indexedbookmarks.com/story18740128/5-essential-elements-for-bip39-explained