In a medium saucepan, combine 2 cups of the mixed berries, sugar, lemon juice, and water. Bring to a gentle simmer over medium heat and cook for 8-10 minutes, stirring occasionally, until the berries break down and the liquid thickens slightly. Remove from heat and let cool completely, about 15 minutes. Reserve the remaining 2 cups of fresh berries for layering.

In a large bowl, beat the cold heavy cream with an electric mixer on medium-high speed until stiff peaks form, about 3-4 minutes. The cream should hold its shape when you lift the beaters.

In a separate large bowl, beat the mascarpone, powdered sugar, and vanilla extract until smooth and creamy, about 1 minute. Gently fold in half the whipped cream to lighten the mixture, then fold in the remaining whipped cream until no streaks remain. Do not overmix or the filling will lose its airy texture.

Pour the orange juice into a shallow dish. Working quickly, dip each ladyfinger into the orange juice for 1-2 seconds per side - they should feel barely moist, not soggy. Arrange in a single layer in the bottom of a 9x13 inch dish, breaking cookies as needed to fill gaps.

Spread half the mascarpone filling evenly over the ladyfingers. Spoon half the cooled berry compote over the filling, then scatter half the reserved fresh berries on top.

Repeat with another layer of dipped ladyfingers, the remaining mascarpone filling, and the remaining compote and fresh berries. Use an offset spatula to smooth the top and create clean edges.

Cover loosely with plastic wrap and refrigerate for at least 4 hours or up to 24 hours. The longer it chills, the more the flavors meld and the cookies soften into cake-like layers. Serve cold, scooping with a large spoon to show off the layers.