For words that have only 3 letters, there's no 4th letter. By way of example with the word “increase”, there are no more letters afterwards and thus has to be the word "incorporate". The word “addict” is in the BIP39 wordlist however, you would have utilised “addi” for “addict”. Various https://altbookmark.com/story21384262/little-known-facts-about-bip39-words