For words that only have 3 letters, there isn't a 4th letter. By way of example with the word “increase”, there aren't any additional letters afterwards and thus has to be the word "add". The word “addict” is inside the BIP39 wordlist but you would have utilized “addi” for “addict”. https://pageoftoday.com/story4327089/the-fact-about-valid-bip39-seed-phrase-that-no-one-is-suggesting