Away3D 4, is a bit more difficult. But I'll try to point you in the right direction.
You need to create a Mesh out of Geometry and Material.
The Material has a texture with your MovieClip.
The Geometry is a PlaneGeometry
The Mesh is the Object3D instance you need to add to your ObjectContainer3D.
You're code would look something like this:
var bitmapData:BitmapData = new BitmapData( mc.width, mc.height, false, 0x000000 );
var texture:BitmapTexture = new BitmapTexture( bitmapData );
var material:TextureMaterial = new TextureMaterial( texture, true );
var geometry:PlaneGeometry = new PlaneGeometry( mc.width, mc.height );
var planeMesh:Mesh = new Mesh( geometry, material );
_3dcontainer.addChild( planeMesh );
Now for the MovieClip to animate, you need to update the bitmap of your texture:
mc.addEventListener( Event.ENTER_FRAME, updateTexture );
private function updateTexture( e:Event ):void {
texture.bitmapData = new BitmapData( mc.width, mc.height, false, 0x000000 );
}
Again, I did not test this, and it's long ago since I used Away3D 4. Just don't forget to import the used classes.
Just have a good look in the docs:
http://away3d.com/livedocs/away3d/4.0/