Oh mate, don't take the above as meaning that the lens you bought is not sharp. You will be most happy with it. Guaranteed. These comparisons are really pixel-peeping and nit-picking over the tiniest little thing. You have bought a great lens that will serve you very well for a long time.
"The Canon EF 70-200mm f/4 USM L IS is the very best zoom lens tested to date in Canon mount. The resolution figures (center/border) at all focal length and all tested aperture settings are nothing short of stunning and also as good as the best fix-focals in this range. At f/11 there´s the usual decline in quality due to diffraction - this is a physical limit. Truly remarkable. "
But the reality is all the Canon 70-200 lenses are exceptional and your selection is based on intended purpose/price rather than ultimate sharpness.
For astrophotography I haven't seen anything done with the 70-200 4 IS yet, so you might be the "guinea pig" Matt. I know the 70-200 2.8 and 4.0 non-IS are excellent lenses for astrophotography, and I've seen mixed comments about the 70-200 2.8 IS lens.