Prepare a medium-high (425°F to 450°F) gas or charcoal grill fire.
In a small bowl, combine the five-spice powder, 2 tsp. salt, and 1 tsp. pepper. Lightly brush the tenderloins with oil, then sprinkle with the spice mixture. Brush the pepper quarters with oil and season with salt and pepper.
Grill the tenderloins, turning a quarter turn every 3 to 4 minutes, until an instant-read thermometer inserted in the thickest part registers 140°F to 145°F, 15 to 18 minutes.
Meanwhile, grill the peppers skin-side down until grill marks form on one side, about 5 minutes. Flip the peppers and continue to grill until crisp-tender, about 2 minutes more.
Let the tenderloins rest for 5 minutes. Thinly slice the pepper quarters.
In a medium bowl, combine the rice vinegar, chili garlic sauce, brown sugar, and garlic. Stir in the peppers and scallions.
Cut the pork crosswise on a diagonal into 1/2-inch-thick slices and serve with the relish.