When CTE is maximized (on a 24" monitor), the grid on the left side occupies only one sixth max of the screen, which is too narrow particularly with long segments. Is this the limit set by design for Layout 4?
You should be able to drag the divider between panes. It can be as wide as the minimal size (which is full visibility) of the toolbars above the source and target editors.
In my (Windows) installation, Layout 4 is set to left-right source and target editors, but I cannot drag their dividing line either.
When I open CTE, the grid pane occupies approximately one sixth of the entire screen when CTE is maximized, and I could shrink it further, but not expand it. Could the reason be that my embedded Java is still version 12?
The possible minimal sizes of segment editors (especially if you switched from the default top/bottom to left/right) may prevent you from increasing the size of the grid. Any increase in the width of the grid would decrease the minimal width of the segment editors, thus hiding the buttons in the toolbars there.
If you hide the editor toolbars (via View > Toolbars > Hide toolbar), you should be able to increase the size of the grid in this case.
I've switched to the top/bottom editor (I don't like it much, though) and things have improved. Below the editors I always keep too panes always opened (I wish they were three), and the general glossary, or a vertical stack of two glossaries, are tabbed to window vertically (on the right side).
I'll experiment with this layout and see if I can get used to it.