For words that have only three letters, there isn't a 4th letter. As an example with the word “increase”, there won't be any additional letters afterwards and therefore should be the word "increase". The word “addict” is inside the BIP39 wordlist however , you might have used “addi” for “addict”. https://hylistings.com/story19916766/the-fact-about-bip39-seed-phrase-word-list-that-no-one-is-suggesting