For words that have only three letters, there's no 4th letter. For example with the word “increase”, there won't be any additional letters afterwards and so need to be the word "insert". The word “addict” is during the BIP39 wordlist however, you would've employed “addi” for “addict”. A seed phrase, https://thebookmarkplaza.com/story18740487/little-known-facts-about-bip39-words