Have you tried File > Import Photos and Video, rather than synchronizing?
Have you tested importing into a new catalog?
Adobe doesn't care any more about LR 4. But you might try downloading the free trial of LR 5 and seeing if it has the same problem. (It won't affect your LR 4 catalogs). If it does have the same problem, then file a bug report at Adobe's official feedback site:
http://feedback.photoshop.com/photoshop_family/products/photoshop_family_photoshop_lightro om