I don't know about v4, but on v3 you can assign it with external utility: autohotkey.com (windows), hammerspoon.org (mac).

With ahk v2 it is:

#if WinActive("ahk_class MoiMainWindow ahk_exe MoI.exe") 
and not WinActive("ahk_class #32770")
!MButton:: Send "5" ;SetNearestOrthoView.js

And bind 'SetNearestOrthoView.js' to '5' in moi's shortcut keys.