Time to enrich the dough! Add 350grams of flour and the salt/sugar water to the risen dough. Slowly pour in the egg mixture, while kneading. Keep kneading the dough until it is elastic and mixed. This probably takes about 10 minutes. The dough should easily get off your hands.
Once all the four eggs are added to the dough, you can add the butter. I love to add the butter in parts so it incorporates easier in the dough. Moreover, a technique I love to use, is to fold the dough over the butter.
Add the dough to a bowl, sprinkle some flour on top and cover with a damp tea towel. Let the dough rise for 6 hours on room temperature. If the room is warmer than 21 degrees Celsius 5 hours will do, if colder than 19 degrees Celsius 7 hours are needed.