all 4 slot ✓ PC won't boot all 4 RAMs

You can install two 8GB RAMs of the same frequency instead of installing one 16GB RAM in one slot If your motherboard and processor support the dual-channelall 4 slot,Let's use the 16-lane slot configuration to put the speed differences between PCIe and into perspective and make all this computer lingo

My question is, should my new stick go into slot #4 or #1 so as to have dual channel all 4 slot,Having four identical apple OEM rams but the system won't turn on with all four slots installed It will with 3, with 2 but not with 4 Also, it

all 4 slot,My question is, should my new stick go into slot #4 or #1 so as to have dual channel