Beyond the sweet addition pictured above, this is the biggest update since 3.x:

• Improves performance when opening large libraries.
• Addresses performance issues when exporting heavily-adjusted images, particularly those with Highlights & Shadows, Definition, Noise Reduction, Edge Sharpen, Sharpen, Skin Smoothing, or Blur.
• Addresses performance when using Core Image filters such as Curves, Levels, and Highlights & Shadows.
• The application now displays a progress dialog when opening or switching between libraries.
• Addresses quality issues in connection with rendering of images scaled to below 100% in the Viewer.
• Fixes an issue that could cause images exported as TIFFs to have an incorrect dpi setting.

Download from software update, or manually here