Whisk 3 tablespoons olive oil together with vinegar, thyme, and honey in a medium bowl. Season to taste with salt and pepper. Add plums and toss to coat; transfer plums to a plate.
Build a medium-hot fire in a charcoal grill, or heat a gas grill to high. Brush kale leaves with remaining 1 tablespoon oil; season with salt. Grill kale, turning once, until crispy and charred at edges, about 2 minutes. Transfer to a work surface; let stand until cool enough to handle. Remove large center stems with a knife and discard (just trim the tough ends from smaller, more tender kale stems). Alternatively, preheat oven to 400°F. Toss kale leaves with oil and arrange in a single layer on a foil-lined baking sheet. Bake for 5 to 7 minutes or until crispy.
Divide ricotta among plates; season with salt and pepper. Stir vinaigrette again. Tear larger kale leaves into pieces (leave smaller leaves whole). Place leaves in a large bowl and toss with some of the vinaigrette. Divide leaves among plates. Top with plums and drizzle some vinaigrette over.