Preheat oven to 350°. Toast hazelnuts on a rimmed baking sheet, tossing once, until golden brown, 8–10 minutes. Let cool, then coarsely chop.
Meanwhile, prepare a grill for medium heat. Cut kernels from 1 corn cob into a large bowl. Add garlic, orange juice, vinegar, and lemon zest and toss to combine; season with salt. Set aside.
Brush remaining 3 ears of corn with oil and grill, turning occasionally, until very tender and charred and blackened in spots, 10–12 minutes. (Alternatively, remove kernels from cobs and cook in a dry large skillet over medium-high heat, turning occasionally, until charred in spots, about 4 minutes.) Let cool.
Cut kernels from cobs and add to reserved salad; add hazelnuts, Pecorino, mint, tarragon, and Aleppo-style pepper and toss to combine. Season with salt.