You can use the localCorners variable of UIRect (of which UIPanel inherits) to get the Vector3 point of any of the corners.
UIPanel mPanel;
Vector3 bottom_Left = mPanel.localCorners[0];
Vector3 top_Left = mPanel.localCorners[1];
Vector3 top_Right = mPanel.localCorners[2];
Vector3 bottom_Right = mPanel.localCorners[3];