That is also my understanding.
Adding the 4th DIMM will drop you down to single channel mode and your bandwidth will tank.
Zip, I am fairly certain having 4 RAM slots on the motherboard instead of 6 was a decision of the motherboard manufacturer not Intel.
It was probably done as a cost saving measure for low tier boards although I am at a loss as to why they would not have just had 3 slots
Possibly they recycled the basic layout from a previous board to save tooling cash??