The "4.5:1 requirement" can be found in the Web Content Accessibility Guidelines (WCAG) 2.0, part of a series of Web accessibility guidelines published by the W3C's Web Accessibility Initiative:

http://www.w3.org/TR/WCAG/

Here is a detailed explanation:

http://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contr...