As far as I understand given a certain latitude the length of a degree of longitude is constant.
So using any distance calculator (I use Mathematica here) this equals: (circumference of the earth)
GeoDistance[{0, 0}, {0, 1}]*360 = 40075km
GeoDistance[{0, 0}, {0, 90}]*4 = 40075km
However, if we change the latitude to 15° for example, this happens:
GeoDistance[{15, 0}, {15, 1}]*360 = 38718.1km
GeoDistance[{15, 0}, {15, 90}]*4 = 38372.7km
Are these roundoff errors? Which one is correct?