Suddenly, photos look different in library module than in develop module. It has to do with red. in library module the red is more orange and more dull. I reinstalled Lightroom 4. No improvement. The difference is dramatic, unprecedented, and intolerable.
Thanks for the suggestion. There's no change in the develop/library rendering difference at 100%. I think what's happening is a particularly dramatic example of the difference between the way an image is rendered in the ProPhoto RGB space of the develop module and the sRGB color space of the library module. In the develop module in soft proof mode I can see that the red is out of gamut for sRGB. By desaturating it I can bring it into sRGB gamut at which point it looks just like the way it's rendered in the library module. So, I think the mystery is solved. The only thing that seems odd to me is that I never, in all the years I've used Lightroom, noticed such a difference before.