For words that have only 3 letters, there's no 4th letter. By way of example with the word “increase”, there isn't any additional letters afterwards and so need to be the word "insert". The word “addict” is while in the BIP39 wordlist however you would've utilised “addi” for “addict”. Standard https://socialistener.com/story4961187/a-review-of-bip39