Just a bit of an update:
Cameras all have been working 24/7 since they are connected to a PoE injector (7 or 8 days now) (
link). So, thanks for your suggestion, it appears indeed that it's a power supply related problem.
I still would like a solution to change the PW to admin/admin, as every time I disconnect and reconnect the lan cables, the original 2 cams (admin/admin) work immediately, the two new ones (admin/8 character PW) I have to manually input the PW.
If ever there's a power outage, the system will reboot and only 2 cams will work. Perhaps not that big of a deal, but it is an inconvenience.
For this, I tried manually hardware resetting the cams, accessing them through Pale Moon (doesn't connect to the ip address of the cam), the Dahua ConfigTool (doesn't allow a 5 character PW), and the SmartAdd function of the NVR (doesn't work at all). All suggestions appreciated!