can I set LR4 to display in sRGB instead?
Easily, that is what soft proofing does. Check the Soft Proof checkbox and select sRGB.
BTW, in the Library module the previews are in Adobe RGB and in the Develop module they are in a variation of ProPhoto RGB. Of course, in both modules the display data has been remapped to your monitor space.
However, and it's a big however, I just don't think Windows Photo Viewer (Windows 7 version) is very reliable. I recently did a comparison between four color managed applications that I use regularly and WPV. The four were: Lightroom 4.1, Digital Photo Professional 3.11.26, Picture Window Pro 5.0 and Breeze Browser Pro. I chose a photo with strong red roofs, blue sky and puffy white clouds and medium green foliage, which I exported from LR in sRGB, compression 85 jpg, no resizing or output sharpening. I then imported the jpg into LR and displayed it on all five applications. The image was close to identical on the four listed above, with slight variations that would only be noticed in side by side comprisons. In Windows Photo Viewer the blues and greens were reasonably close, but the reds were strongly desaturated and looked terrible. Colors with a strong red component (some yellow flowers, for instance) were similarly desaturated and hue-shifted. Overall, the image was contrastier, with the resultant illusion of greater sharpness.
My conclusion was to stay far away from WPV.