Grate the skin of two of the lemons and squeeze the juice from all four lemons. Set aside.
In a heat proof bowl; whisk together the eggs and sugar until the sugar is dissolved.
Add the lemon juice and lemon zest.
Put the mixture in a double boiler or a bowl over a pot of simmering water.
Add butter cut into small pieces.
Cook over medium heat, stirring often, until the lemon curd thickens; this may take up to 30 minutes. Be sure to keep water in the bottom pot during the cooking time.
Refrigerated, this curd will keep for up to 1 month.