For words that have only 3 letters, there's no 4th letter. As an example with the word “increase”, there are no extra letters Later on and therefore have to be the word "incorporate". The word “addict” is inside the BIP39 wordlist however, you would have employed “addi” for “addict”. Safety: https://ledbookmark.com/story4501880/the-greatest-guide-to-seed-phrase-bip39