1. In a food processor or bowl, combine flour and butter. Whirl or rub in with your fingers until fine crumbs form. Add 1 egg and whirl or stir with a fork just until dough holds together. Pat into a ball, then press evenly over bottom and up sides of a 9- to 10-inch tart pan with a removable rim.
2. Bake in a 325° oven until crust is pale gold, 25 to 30 minutes. Let cool.
3. Meanwhile, in a bowl, whisk together remaining 3 eggs, grapefruit peel and juice, sugar, and the 2/3 cup whipping cream. Set baked crust on oven rack and carefully pour in filling.
4. Bake in a 350° oven until filling jiggles only slightly in the center when pan is gently shaken and tart is golden at edges, 25 to 30 minutes.
5. Let tart cool on a rack at least 45 minutes. Serve slightly warm or chilled, with sweetened whipped cream and grapefruit segments.