Both your NVR and cameras are quite old and have been end of life for some time. The 7604NI-SE-P can take cameras up to 5MP resolution. Both the cameras and NVR only support H264 encoding.
I'd probably recommend updating the NVR before the cameras. Swapping it out for a 7604NXI-K1/4P would be the logical replacement. That model:
- supports H265 encoding which is more efficient on disk space (once cameras are updated as they also need to support H265).
- accepts 4 x 4MP cameras
- supports Hik-Connect app and service (your existing NVR only supports the old HiDDNS service which is no longer available)
- has AcuSense which will allow you filter motion detection events to Human for fewer false alerts
- as an up to date model it's still receiving firmware updates so in the event of s security issue can be patched (there's a very good chance your cameras are on old firmware and will be accessible using 'admin' and 12345 as the password)
Once the recorder is replaced you could replace cameras as you like. I'd recommend the DS-2CD2347G2H-LIU:
- Turret format - won't collect rain residue like your current domes
- 4MP
- large 1/1.8" sensor - ultra low light
- Hybrid infra red and white LED illumination - can be set to normal colour/mono, 24/7 colour or smart (colour/mono but switches back to colour in darkness when an event is detected)
- AcuSense false alarm filtering. While it's available on the NVR detailed above, using it on the camera (where available) provides better results.