You can run Gantry 5 alongside Gantry 4, yes. But you can't use them in the way you described. A menu item has to be using Gantry 4 or Gantry5. You can't have mobile view on Gantry 5 and desktop view on Gantry 4 for the same menu item.
I would also say that trying to use both Gantry 4 and Gantry 5 at the same time can get awfully complicated so it's best to stick to one or the other.
Regards, Mark.
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