<image>To ensure secure information transmission, there is a type of cipher system called a secret key cryptography system. Its encryption and decryption principles are shown in the following diagram: 

Now the encryption key is \( y = \log_{a}(x+2) \), as shown below. The plaintext "6" is encrypted to get the ciphertext "3", and then it is sent. The receiver uses the decryption key to decrypt the ciphertext back to plaintext "6". The question is: if the receiver gets the ciphertext "4", then the plaintext after decryption is ______ .