For words that have only 3 letters, there's no 4th letter. As an example with the word “increase”, there isn't any more letters afterwards and so has to be the word "include". The word “addict” is while in the BIP39 wordlist however, you would have used “addi” for “addict”. Meticulously https://businessbookmark.com/story4933478/the-fact-about-bip39-seed-phrase-word-list-that-no-one-is-suggesting