This is what you can expect to get with a analog DVR.
I have been there with 4 analog cameras in the car, and it was a waste of my money.
I can only recommend to NOT go with analog systems of low resolution, and while i have not tried it out myself i now know enough to also say i can NOT recommend people going with low bitrate 1080p systems, you will want at least 9mbit bitrate on each channel, 15 - 18 mbit even better.
Say 4 cameras recording 1080p at 15 mbit that will yeld a need for writing of at least 1.2 Gb of data every 3 minutes,that's around 24 Gb for 1 hour or recording, so you would have about 5 hours of recording on a 128 Gb memory card,,, if you can find a DVR that support that size or dare to take a chance it can be made to do so by formatting large sd card with fat32
You could also buy a home CCTV IP camera NVR and you can fit 4 1080p cameras to it and at least mine claim ot can handle a total of 80 mbit incomming data from those 4 cameras, and you can buy small IP cameras too just fine, but as i am also walking that avenue with my home CCTV there are no way in hell i would try to do that in my car.
BUT ! if i hit it big in the lotto i might try that on my dream car as it will be a 15 tonne bulletproof monster, but then i would not use budget NVR or cameras like i do at home now.
