For words that only have 3 letters, there is no 4th letter. As an example with the word “increase”, there aren't any additional letters afterwards and so has to be the word "add". The word “addict” is in the BIP39 wordlist however you would have employed “addi” for “addict”. A https://blogfreely.net/travenmdoq/h1-b-bip39-word-listing-explained-your-important-source-for-creating-secure