As you pointed out there isn't much difference. But you are only looking at driving in top gear. Hills, wind, weight etc will cause downshifting. What's the difference between 5th and 4th on the 5 speed compared to 4th and 3rd on the 4 speed?
My personal belief is for a cube van like the Unicell CW to attain decent fuel mileage, a 6 speed transmission and an axle ratio no lower than 3.73 is required. I did the math and some guess work when specing a Unicell CW on a Chevrolet chassis with a 6.0 and the above drivetrain. My driving habits and those specs, I concluded that mpg in the high 16's was doable for the life of the van, all miles.