I have the 4gb in slots 1 & 3 ( which is also where Apple had them pre configured) and the new 8GB in slots 2 & 4 to get optimum performance. EDIT********* I have since purchased a 2019 5k iMac 27” 3.0GHz 6-Core and tried to use the 16gb 2400 MGH and it was down throttling the ram.
https://www.amazon.com/Crucial-2x16GB-Laptop-Memory-CT2K16G4SFRA32A/dp/B08C4X9VR5