Black screen when using 4 ram sticks on brand new iMac 5K
Not all RAM is created equal. The way motherboards handle things is radically different. For example where an ASUS PC motherboard might take mismatched RAM, the iMac will not. To ensure compatibility use RAM that has the same:
- Clock Speed
- Voltage Requirement
- Latency timings
It's best to buy identical RAM sticks. They should be the same manufacturer and the same model number. Occasionally even RAM with the same model number can have different modules on the chip. (This is very rare )
Can you update your question with details about the Manufacturer, Model number, Clock speed, voltage Requirement, and latency timings of your RAM?
UPDATE: According to the Apple website, the RAM you bought should be compatible: https://support.apple.com/en-us/HT201191
Might be time for the genius bar.
If, as you say, each of the sticks works individually, but not altogether, then I would suspect the logic board to be at fault.
Also, from personal experience today: I got a black screen on my 2018 Mini, because I hadn't seated the new sticks correctly. The new Macs with T2 chips don't make memory error beeps at startup.
It still might be that the RAM isn't seated correctly, even if you pushed hard. Upgrading my older Mac, I got the exact RAM specified by Apple as being compatible.
Following the directions on the Apple support site, I felt each RAM chip click. But nothing afterwards.
Finally in a fit of frustration, I hammered all of them in with the back of the screwdriver handle and it worked perfectly. Percussive maintenance isn't usually recommended for Macs but in this case it worked.