For words that have only three letters, there is not any 4th letter. By way of example with the word “increase”, there isn't any much more letters afterwards and thus needs to be the word "insert". The word “addict” is inside the BIP39 wordlist however, you might have utilised “addi” https://meshbookmarks.com/story19089825/invalid-bip39-phrase-things-to-know-before-you-buy