I haven't tested it, but as a ExtJS4 chart always needs a surrounding panel with layout: fit or any other layout assigned which determines the size of the chart, I would say it should work in an ExtJS 3 window with layout:'fit' and given width/height and then chart added as items: [{YourExtJS4ChartObject}]
I am fancing an issue. I have rendered ExtJS 4.0 charts in ExtJS 3 application using sandbox. But I am not getting chart displayed in IE. It works fine in FF and Chrome.
I have put tooltip on column chart. So, when I move cursor to area where actually chart should be displayed (As I told you chart is not getting displayed in IE), it show me tooltip but chart is not visible. It means it actually renders chart but cannot view it.