Preheat the oven to 350 degrees F (175 degrees C).
Heat butter in microwave oven in a 4-cup microwave-safe measuring cup until melted but not hot, about 1 minute. Beat eggs, sugar, and flour into butter until smooth. Spread rhubarb in unbaked pie crust; pour egg mixture over rhubarb.
Bake in the preheated oven until filling is lightly browned, about 45 minutes.
