And I was wasting time trying to do the same with one frame-container (not possible with pack?).

No, I dont think its possible,

the docs say Tk::pack - Geometry manager that packs around edges of cavity, a geometry manager that arranges the children of a parent by packing them in order around the edges of the parent.

Once the Panes are arranged on the edges, two on top and two on bottom, there is no option for them to be expanded half way, so they'll either be stacked vertically or horizontally