1. MAKE THE CRUST: Preheat the oven to 325°. In a medium bowl, mix the graham cracker crumbs and melted butter. Press the crumbs evenly over the bottom and up the sides of a 9-inch glass pie plate. Bake the crust for about 15 minutes, or until firm. Let cool.
2. MAKE THE FILLING: In a large bowl, whisk the 3 egg yolks with the lightly beaten whole egg until thickened. Beat in the condensed milk, then beat in the lime juice and zest.
3. In a large stainless steel bowl, beat the egg whites until firm peaks form. Fold one-third of the beaten whites into the custard, then fold in the remaining whites. Pour the custard into the pie crust and bake for 15 minutes, or until barely set. Transfer to a rack and let cool completely, then refrigerate over-night. Serve with sweetened whipped cream and garnish with lime zest.