When both slots are populated, the system only records 2GB of memory being available. I have tested both RAM slots by...

@Louis Rossmann gave the following answer (which has subsequently been deleted) which I actually found useful (my solution is to max out top slot)! I'll place it as a comment and remove if it is found factually inaccurate. Bottom slot is soldered using BGA soldering, top slot is soldered using through hole soldering. It is common for the bottom slot to die. The pads slowly come away from the board or disconnect from internal vias, usually from a drop or from someone soldering near the slots with the RAM still attached. There is no fixing this, just max out the top slot and call it a day, and thank Apple for using BGA for soldering a RAM slot to the motherboard.