May 1, 2010 is what the Environmental Protection Agency--the federal bureau that oversees the Energy Star project--calls the new requirements' "effective date." No televisions manufactured on or after that date will be able to carry an Energy Star logo unless they are 4.0 compliant. If a particular model is only 3.0-compliant, a unit built on April 30 could have the logo, but not one built the next day.
In addition, such models will disappear from the of compliant televisions as of that date. Meanwhile, the EPA's list will continue to be updated with 2010 models that are compliant.
The outgoing 3.0 specifications were not particularly strident. A 50-inch set could consume a full 318 watts when turned on and still get to display the logo.
By comparison, Energy Star 4.0 means a 50-inch HDTV will not be able to carry an Energy Star logo if it burns more than 153 watts--just shy of half 3.0's maximum.