Preheat the air fryer to 425ºF. On a baking sheet, season the pork tenderloin all over with salt and pepper, then rub with 2 tablespoons of the oil. Transfer the pork to the air fryer and cook until golden brown and an instant-read thermometer stuck into the thickest part reads 135ºF to 140ºF for medium, about 15 minutes. Transfer to a cutting board, tent with foil and allow to rest for 10 minutes.
Meanwhiile, in a large bowl or on a board, smash each potato just enough to break open, but not so much that they fall apart. Drizzle the remaining 2 tablespoons oil over the potatoes. Transfer to the air fryer and cook until golden and crispy, about 15 minutes. Transfer to a serving platter.
While the potatoes are cooking, make the pistachio sauce: Add some of the pistachios to a mortar and pestle and pound until you have a mixture resembling very small crumbles. Transfer to a serving bowl. Add the remaining pistachios and the garlic; pound until ground. Transfer to the bowl. Add the lemon juice, orange zest and brandy; mix well. Add 1/4 cup of olive oil, parsley and tarragon; mix well again. Taste and season with salt and pepper.
Slice the pork tenderloins into ½-inch medallions and arrange on the platter with the potatoes. Generously drizzle the sauce over the top. Toss the arugula with red wine vinegar and the remaining 2 tablespoons of olive oil, season lightly with salt and pepper, and serve alongside.