Have you tried it with only two sticks installed? In the slots that are second and fourth away from the CPU socket (A2 and B2 for most boards. A few boards might call them DDR4_1 and DDR4_2, but it will be the second and fourth away, for all modern boards)?
See if you still have the same problem with only two sticks installed? If you do, try swapping out one of the sticks for one of the two not currently installed. Problem still? Swap out the other stick for the remaining stick that hasn't been tried yet? Still a problem, likely the motherboard.
Before you do any of that though, make sure you have the MOST recent motherboard BIOS version installed, which would be version 1302 for the Prime Z270-A and which specifically addresses improvements in memory compatibility and performance.
Also, make sure that you have the LATEST available chipset drivers installed, which would be these:
https://downloadcenter.intel.com/download/28182/Chipset-INF-Utility?product=98089