There are some pretty good deals available right now...

You can currently get a 4-camera ANNKE H800 8MP/4K PoE Security Camera System for $450 from Amazon (after $80 coupon, see link below). This system comes with 4 x 8MP/4K/PoE dome cameras (Hikvision OEM, maximum resolution possible at this price point) with motion detection, and remote viewing support on your smartphone. Add a fifth additional camera for $76 (after $40 coupon), and up to more 3 additional cameras for $116 each, to a maximum of 8 cameras for this system. There is also a kit that contains the maximum of 8 cameras as described, and a larger hard drive for $880 (after $110 Amazon coupon).

https://www.amazon.com/ANNKE-Security-Cameras-Pre-Install-VandalProof/dp/B08X19L9PW/

https://www.amazon.com/ANNKE-Security-Cameras-Weatherproof-Vandal-Proof/dp/B08YDZWNFJ/

https://www.amazon.com/dp/B08P7NNWR1

I'm a big fan of these cameras, because they are rugged, have a discreet profile, and the video quality is incredibly sharp and crisp. Each camera has a rugged metal housing, automatic infrared night vision, and IP67/IK10 vandal + weather resistance. While it's clear for an observer to see that there is a camera installed, unlike some "bullet" or "turret" style cameras, it's not obvious to see exactly where the cameras are pointing unless you are looking very closely at them, which is a big advantage and a nice deterrent in my opinion.

Each camera requires a dedicated Ethernet cable to the NVR. Since each cable will be carrying video signal and PoE voltage to power the camera, make sure to use high quality cable and connectors composed of solid (not stranded) pure copper (not CCA). Here is one example:

https://www.amazon.com/Monoprice-1000FT-350MHz-Ethernet-Copper/dp/B001VPOGYE/

It's a good idea to conceal and/or secure the NVR from theft. In the event of a break-in, you want to avoid having the NVR with footage of the theft stolen along with everything else... Consider using a steel enclosure, which is securely bolted to wall/ceiling studs or floor. For added security, I would modify the enclosure slightly by adding one or more hinged hasps and padlocks to prevent theft.

https://www.amazon.com/Kenuco-Heavy-Gauge-Security-Lockbox/dp/B073DBMNQ1

https://www.amazon.com/dp/B00X6E0HTG/

For convenience and/or theft deterrence in a retail space, it might be convenient to display the live cameras at a different location than where the NVR is installed. In this case, you may want to "extend" the live HDMI signal over Ethernet cable to a different location where a separate display monitor or TV is installed. You can use HDMI extenders for this purpose. There are many different types, the cheapest ones having a shorter range on the distance between the transmitter and receiver. Pricier ones are able to transmit a longer distance, and may allow for transmission to multiple displays simultaneously, if required.

https://www.amazon.com/164-Feet-Extender-Single-Option-EX-165C/dp/B01GYH8DOM/

https://www.amazon.com/Extender-Splitter-Transmission-Loop-Out-Function/dp/B071SHDX7Z/

When working with the PoE cameras and HDMI extenders shown above, each cable should be a home run (dedicated cable) to the NVR or transmitter. More advanced/expensive extenders are available that can share a single cable, however, they should still be completely isolated from any existing network.

Be sure to plan your install carefully. Familiarize yourself with and observe all standard personal safety procedures, applicable code requirements, building/landlord policies and statutory requirements for installing low voltage building wiring, surveillance systems and working with electronic systems.