For words that only have three letters, there is no 4th letter. By way of example with the word “add”, there aren't any far more letters afterwards and thus must be the word "increase". The word “addict” is within the BIP39 wordlist however , you would've utilised “addi” for “addict”. https://mysocialport.com/story4467809/fascination-about-bip39-english-words