2008 Voter turnout was only higher by less than 1% compared to 2004. What made the difference was that GOP voter turn-out dropped nearly 2% and Democrat turn-out rose nearly 3% when compared to 2004.
GOP
2004 = 30 %
2008 = 28.7%
Democrat
2004 = 28.7%
2008 = 31.3%
From American University Center for the Study of the American Electorate [1]