For words that have only 3 letters, there isn't any 4th letter. For example with the word “increase”, there aren't any extra letters afterwards and therefore should be the word "insert". The word “addict” is within the BIP39 wordlist however, you might have made use of “addi” for “addict”. This https://tripsbookmarks.com/story18901340/the-invalid-bip39-phrase-diaries