For words that have only 3 letters, there's no 4th letter. As an example with the word “increase”, there isn't any much more letters afterwards and thus needs to be the word "increase". The word “addict” is within the BIP39 wordlist however, you might have made use of “addi” for https://sb-bookmarking.com/story19546399/the-5-second-trick-for-ledger-seed-phrase-bip39