For words that have only three letters, there is not any 4th letter. One example is with the word “incorporate”, there aren't any additional letters afterwards and thus has to be the word "incorporate". The word “addict” is during the BIP39 wordlist but you would've made use of “addi” for https://www.mediafire.com/file/qad32ryob76naol/pdf-7033-8222.pdf/file