Fubeca: I think it may boil down to speed at which you tested. A 4% gain is respectable at a 55 mph test speed. The faster the test, the larger the percent difference between MPG with aero cap vs. open bed. (Note that Brett & Phil tend to test at higher speeds.)
A rough illustration using the
aero + rr formulas embedded in the calculator:
If I change the Cd of my car from 0.34 to 0.23 (which is within the realm of possibility when all the aero mods I've tried are in place), it tells me I'd see a 31.9% MPG improvement at 50 mph... but a 39% improvement at 75 mph. (Owing to the fact that aero load increases with the cube of speed.)