For words that only have 3 letters, there is absolutely no 4th letter. For instance with the word “increase”, there are no far more letters Later on and thus must be the word "increase". The word “addict” is within the BIP39 wordlist but you would have utilised “addi” for “addict”. https://blogfreely.net/travenmdoq/h1-b-bip39-word-listing-explained-your-important-source-for-creating-secure