Is there any tool that will make it dead-easy to quarter the size of an image (i.e., de-Retina it)?
Automator

You can use OS X's Automator.app to quickly resize images to 1/4 of their original size. The Actions you want are:


Get Selected Finder Items
Scale Images




You can create droplet like applications using Automator to make the whole process a single drag-and-drop. Alternatively you can set up a folder action to resize any image dropped into a folder.

To learn more about using Automator, see Apple's Mac Basics: Automator.