If you could get away with using 4 AA cells without any other circuits that would be the most efficient, however, it’s not guaranteed to work.
The Linear Tech part mentioned earlier definitely has a loss in efficiency when working with lower input voltages, it is however designed to work with higher voltages when you want that sort of current.I part designed specifically for 2 - 3.5 V in 5V out should be much more efficient (although probably not when compared to 4.8V on a device designed specifically around that voltage). I would be surprised to see more than 10% difference between the two optimised solutions.
Okay, there are a huge number of assumptions there so the statement is rather risky.
Testing would be simple. Provided I had a 4x and a 2x unit to test. I have a couple of variable supplies which give current supply reading accurate to 1 mA (give or take only a couple mA from my experience). Use the supply in place of the batteries to charge a mp3 player and check the current draw.
