To replace or augment Sigma 70-200 OS?

I am in a bit of a quandary regarding what to do with my Sigma 70-200 OS. A recent windfall is burning a hole in my pocket and I was contemplating replacing it with the superlative Nikon 70-200 VRII. However, if I sell my Sigma, the gap between what I'll get for it and the cost of the new lens is roughly equivalent to the price of a Nikon 70-200 f4. So now I'm thinking, maybe I should get the f4 then I will have both a fast 2.8 and a light/compact-ish f4. Thoughts/opinions?