<image>A security department uses a key cipher system to ensure secure information transmission. The principles of encryption and decryption are shown in the diagram:

Suppose the decryption key is: x → y = a^x (where a > 0, a ≠ 1). As shown above, if the ciphertext "3" is decrypted into plaintext "8", then when the sender inputs the plaintext "4", the ciphertext that the receiver should obtain is "____".