What is in the 4-pin DC cable?
The images in the eBay post show a 12v 7a power supply with a pinout on the power brick. The pinout shows only a 12v positive (pin) and ground (barrel).

The connector is called a 4P Power Snap and Lock, but you may also find it loosely described as a 4 pin DIN connector. The latter description is slightly inaccurate.

If you have a volt meter, you can confirm that the inside four pins are all 12v. It's not out of the question that all four pins are hot in order to handle the relatively high current capacity of the 7 amp power supply or 7 amp power requirements of the DVR.