For words that only have three letters, there's no 4th letter. By way of example with the word “incorporate”, there are no much more letters afterwards and therefore need to be the word "insert". The word “addict” is within the BIP39 wordlist however, you might have employed “addi” for “addict”. https://userbookmark.com/story19565563/little-known-facts-about-bip39-words