http://casinogambling.about.com/libr...y/aa050103.htm
http://pokerhomey.wordpress.com/2007...le-of-4-and-2/
Here's a couple links I found re: the rule of 4 and 2. It's definitely "accurate" but not necessarily "precise." But if you consider your outs conservatively (I always do) and use implied odds as a final counterpoint to help make a decision, it's a good measuring tool.