Flex 4 didn't replace all mx components. For example there are no spark data grids or charts.
So if there's no spark equivalent you should use mx components.
The spark Panel and Application expose controlBarContent properties:
<s:Button label="Control 1" click="trace('Hello World')" />
<s:Button label="content" />
See more information in the spec: http://opensource.adobe.com/wiki/display/flexsdk/ControlBars+in+Spark+Panels
There is no equivalent tag, but there are controlbars for Spark Panel and
Application. See the doc.
I've seen that, but I'm trying to use one outside a Panel. In Flex 3 I believe you can just use <mx:ControlBar /> where you want. It also now seems to be a property, not a Class unless I'm mistaken.
mx:ControlBar wasn't much more than an Hbox that had special placement in
the skin. So, make a custom skin with a spot for an Hgroup. That's more or
less how Panel does it.