For words that only have 3 letters, there is no 4th letter. As an example with the word “increase”, there are no far more letters Later on and therefore should be the word "incorporate". The word “addict” is within the BIP39 wordlist however, you would have utilized “addi” for “addict”. https://mysocialname.com/story4652073/not-known-factual-statements-about-bip39-explained