Currently you can split the 3D viewer into 4 quadrants which is great, but it still forces you to pick either Orthographic or Perspective (T shortcut) which applies to all Views.  Typically if I'm using 4 split view in 3D, I want to have one perspective view, and 3 orthographic views.  That way I can see the "real" scene, plus can fine edit the exact positioning using the orthographic front/top/side views.

So my suggestion is that the Perspective/Orthographic toggles (as well as the grid) should be per sub-view, not global for all splits. You already have the concept of the "active" view since it highlights the one that is being modified, so just extend to also cover the controls in the view toolbar that are per-view.