In the bowl of a stand mixer fitted with a paddle attachment, beat together mascarpone, cream cheese, 3 tbsp sugar and lemon juice for about 2 minutes, until light and fluffy.
In a separate bowl, use the whisk attachment to whip together the heavy cream, vanilla, and remaining 1 tbsp sugar about 2 minutes until the cream holds soft peaks.
Gently fold together the two mixtures until fully combined. Distribute the mousse into small serving bowls and refrigerate at least 2 hours.