For words that only have 3 letters, there isn't any 4th letter. Such as with the word “incorporate”, there are no additional letters afterwards and therefore should be the word "include". The word “addict” is within the BIP39 wordlist but you would have utilised “addi” for “addict”. On this page https://bookmarklayer.com/story18927865/fascination-about-bip39-english-words