For words that have only 3 letters, there is no 4th letter. One example is with the word “incorporate”, there are no far more letters afterwards and therefore need to be the word "incorporate". The word “addict” is within the BIP39 wordlist but you would have employed “addi” for “addict”. https://yxzbookmarks.com/story19594016/new-step-by-step-map-for-bip39-english-words