One of the most requested features I get is support for the Retina display. The next release of Decked Builder, version 1.4, will be adding preliminary support for HD card images. Unfortunately, I do not have an iPhone 4 for testing (stuck with a 3GS).
If anyone is willing to help beta test the next version of the app, just send an email to deckedbuilder@gmail.com. Thanks!
Decked Builder is unofficial and is not affiliated with, endorsed, sponsored, or approved by Wizards of the Coast. Magic: The Gathering is a trademark of Wizards of the Coast LLC.