There are 2 questions:
Is it possible to specify the width of a panel in the XML layout?
For layoutPanel (panel_rowX_colY), Can I set columns to more than 3?
For question #1:
I don't think you can specify the width of the panel's themselves. You can specify the width / height of the flash charts in the advanced xml.
For Question #2:
I think the max is 4 panels, per this documentation: