Dietary need scales with volume, whereas incident sunlight would scale with surface area.

Assuming a spherical cow and a spherical human, the calories needed would scale with the radius cubed, whereas the calories gained from sunlight would scale with the radius squared. So while I agree this wouldn't be very many calories, even if you sat under the sun all day, I think the 4% figure is probably quite pessimistic.

Mmm... Cows weight like 5 persons, 5^(1/3)=1.7, so 4%*1.7=7%, so 170 kcal that is still less than a single cheeseburger.

Marvelous...

Finally a good reason to assume a spherical cow!

The square cube law pops up all over the place, my favorite being in fusion reactors, where it is a two edged sword.