For words that have only 3 letters, there is absolutely no 4th letter. As an example with the word “incorporate”, there are no extra letters afterwards and thus must be the word "incorporate". The word “addict” is in the BIP39 wordlist however, you would've made use of “addi” for “addict”. https://socialnetworkadsinfo.com/story19971022/fascination-about-invalid-bip39-phrase