For words that only have 3 letters, there is no 4th letter. For example with the word “incorporate”, there aren't any additional letters Later on and therefore has to be the word "increase". The word “addict” is while in the BIP39 wordlist however you would have utilised “addi” for “addict”. https://thebookmarkage.com/story19449853/little-known-facts-about-bip39-words