Whisk 3 Tbsp. oil, vinegar, thyme, and honey in a medium bowl. Season vinaigrette 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 Tbsp. 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).
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.
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