I have mounted 4ch DVRs on rally cars. On that kind of installations, you have to keep several things in mind, some of them already stated on this thread.
First, weight. If possible, you do NOT want to add 5Kg of weight to the car for a few cameras, the lighter, the better.
Then, shakiness. Depending on the kind of car, normal HD is not an option. You can use SSD HD, you can use SD memory cards which are very cheap, etc, but not a normal hard drive.
The lightest one I have mounted weights 0.7Kg, it can record 4 channels A/V and uses SD cards which can easily be replaced. It gets power from ignition, you can easily set it up so it will just turn on when the car starts. The MDVR powers the cameras, so you run wires for power and video from the DVR, to every place where you will mount a camera.