I'm affraid that carmax don't sell BMW Z4 Roadsters. They sell other kinds of BMW Z4 though, including convertible types. I think it will depend on if they can get any in second hand.
The 2012 BMW Z4 has 16 valves.
The 2005 BMW Z4 has 24 valves.
The 2009 BMW Z4 has 24 valves.
The 2008 BMW Z4 has 24 valves.
The 2011 BMW Z4 has 24 valves.
The 2010 BMW Z4 has 24 valves.
The 2014 BMW Z4 has 16 valves.
The 2003 BMW Z4 has 24 valves.
The 2007 BMW Z4 has 24 valves.
The 2006 BMW Z4 has 24 valves.
The 2013 BMW Z4 has 16 valves.