For words that have only 3 letters, there's no 4th letter. As an example with the word “increase”, there isn't any much more letters afterwards and so must be the word "incorporate". The word “addict” is inside the BIP39 wordlist however you would have made use of “addi” for “addict”. https://bookmarkshq.com/story20528906/not-known-factual-statements-about-bip39-explained