Ram Slot allocation for iMac 2010

Lets see if we can clear a few things up here.

Memory upgraded in matched pairs is ideal. While you can use mixed modules (size wise) I don't recommend using the same size from two different specs. Here is a good reference while it is on the older G5 series iMac it still applies today! Benefit of using matching memory modules (RAM). Remember your system has a 64/128-bit memory bus. I would recommend you get a second 4GB module if you really want the benefit of faster access.

Banks 0 & 1 & Odd/Even modules:

When you place modules you want to have the bank Top & Bottom (Bank0 & Bank1) have the same modules within the two slots each bank has, Left & Right (Odd & Even).

Here is the Apple instruction sheet for installing memory, you'll need to jump down to the diagram to get a better idea what I'm speaking about: iMac: How to remove or install memory.