Flex4 namespace

Digested from [ref]

• MXML 2006: The legacy MXML language namespace used in previous versions of Flex. Flex 3 applications compiled using Flex 4 can continue using this namespace.
  URI: http://www.adobe.com/2006/mxml 
  Default Prefix: mx
• MXML 2009: The new MXML language namespace. This is purely a language namespace,
  and does "not contain component tags"?.  URI: http://ns.adobe.com/mxml/2009 
  Default Prefix: fx
• Spark: This namespace includes all of the new Spark components. It should be used in conjunction with the MXML 2009 language namespace. URI:  library://ns.adobe.com/flex/spark  ?
  Default Prefix: s
• MX: This namespace includes all of the MX components. It should be used in conjunction with the MXML 2009 language namespace. URI: library://ns.adobe.com/flex/mx  ?
  Default Prefix: mx

[ref]
http://www.adobe.com/devnet/flex/articles/flex3and4_differences.html#_Namespaces_and_packages