For words that have only 3 letters, there isn't any 4th letter. For example with the word “incorporate”, there are no more letters afterwards and so must be the word "add". The word “addict” is during the BIP39 wordlist however , you would have utilized “addi” for “addict”. An easy https://socialexpresions.com/story4462388/the-invalid-bip39-phrase-diaries