For words that only have 3 letters, there isn't any 4th letter. By way of example with the word “add”, there isn't any a lot more letters afterwards and therefore has to be the word "add". The word “addict” is in the BIP39 wordlist however , you would've applied “addi” https://bookmarkja.com/story21146831/invalid-bip39-phrase-things-to-know-before-you-buy