Preheat oven to 350°F (175°C). Melt 4 tbsp butter and pour into a greased 10-cup Bundt pan, tilting to coat. Sprinkle brown sugar evenly over butter, then arrange pineapple rings and cherries in a single layer.

Beat ½ cup room-temperature butter and granulated sugar on medium speed until pale and fluffy, about 3 minutes.

Whisk flour, baking powder, baking soda, and salt in a bowl. In another, mix eggs, vanilla, and buttermilk. Add dry and wet mixtures alternately to the creamed butter, mixing just until combined.

Spoon batter over the fruit and spread gently. Tap the pan once on the counter.

Bake 45 to 50 minutes until a toothpick comes out clean. Cool in the pan for 10 minutes, then loosen edges, invert onto a plate, and serve warm.