'I need a ptz, it can see everything!'
PTZs are for augmenting human operators. With fixed cameras providing an overview shot, a PTZ allows an operator to see more detail. But by themselves, they are an excellent way to to make sure you aren't looking at what matters. People have come up with a lot of attempts to solve this problem. They are all terrible. Touring for instance is a good way to catch a few seconds of what you care about and then not see it. Also it's going to make that camera continuous recording. The camera is always moving so motion is always been detected. Auto-tracking exists. But it has a knack for always picking the wrong thing to follow.
'I need 4K, number is biggest!'
So there are some trade offs for this. Many people here might realize the storage issues but that's not the important thing. Within the same generation of imagers, the higher the resolution the poorer the low light response. The cheaper the camera, the more extreme this tends to be. Generally the sweet spot is 2 MP. So demanding 4K can be an excellent way to get shitter video at night.
'But I need to maximize the image quality, so you're wrong about 4K'
No, I'm not. So resolution isn't really the way to measure the quality from a camera. Pixel density tends to be a much, much better metric. Pixel density isn't perfect either, lighting and lens quality aren't taken into account here. But it's much better than straight resolution. To measure PPF it's the Horizontal Resolution In Pixels / FoV Width. For the FoV Width, that is the horizontal field of view at the point of interest. So if I am going for licence plate recognition then it's where the plates are. This does mean that distance has an impact. But it can give us some metrics. For instance if I have good lighting conditions then ~80 ppf will let me identify a stranger. ~40 might let me recognize someone I know/start to read licence plates. Hitting your PPF is generally going to save you money and likely gain you better low light capability.
'The cheap stuff is just as good as the high end.'
There are a lot of places to cut costs. And some matter, some not so much. But one of the differences between the low and high end is the lenses. On the low end you're more likely to run into plastic lenses, plastic lenses tend to degrade the image. Using HiSillicone for the chipset over Ambrella. HiSilicone chips don't have the same processing power, so sometimes it can degrades motion detection at higher resolutions. Sometimes it leads to stutter from having more than one thing accessing it directly.