Preheat a grill to medium high. Combine the scallions, cilantro stems, jerk seasoning, garlic, lime zest and 2 tablespoons olive oil in a food processor. Pulse, scraping down the processor a few times, until a coarse paste forms. Toss the chicken with the paste in a medium bowl.
Lay out four 16- to 18-inch-long sheets of foil. Divide the sweet potatoes among the foil and drizzle with the remaining 2 tablespoons olive oil. Season with salt and pepper and toss well. Arrange the potatoes in the middle of the foil and top with the chicken. Bring the two short ends of the foil together and fold twice to seal; fold in the sides to form a packet.
Grill the foil packets, turning once, until the chicken and potatoes are cooked through and lightly browned, about 20 minutes.
Meanwhile, make the salsa: Roughly chop the cilantro leaves and radishes; transfer to a medium bowl. Peel, core and chop the pineapple; add to the bowl. Add the lime juice, a few shakes of hot sauce and a pinch each of salt and pepper; toss well.
Carefully open the foil packets. Top with the pineapple salsa.
Be sure the potatoes aren’t too thick — otherwise they won’t be fully cooked when the chicken is done.