Preheat oven to 350 degrees.
Break 2 bananas into pieces and puree in a blender with lemon juice, granulated sugar, and yolks until smooth. In a very small saucepan heat cream over moderate heat until hot and add banana mixture, stirring until combined well. Divide mixture between two 2/3 cup ramekins. Put ramekins in a metal baking pan and fill pan with enough boiling hot water to reach halfway up sides of ramekins. Put pan in middle of oven and lay a sheet of foil over each ramekin. Bake custards until set, about 30 minutes.
Set broiler rack so that custard tops will be 1 1/2 to 2 inches from heat and change oven setting to broil. Alternatively, have ready a blowtorch.
Just before serving, very thinly slice the 2 remaining bananas and arrange slices, overlapping them, over each custard. Sprinkle turbinado sugar evenly over bananas and heat under broiler (or with blowtorch, moving flame evenly back and forth just over sugar) until sugar is melted and caramelized.