Place 3 cups (400 grams) of the flour, sugar, salt, and instant yeast into the bowl of a stand mixer.
In a saucepan, melt the butter and milk together until mixture is very hot. Let it cool down to about 130 degrees (measure with a thermometer for accuracy).
Stir the milk into the flour mixture, then add the egg and stir until mixed. You can do this by hand or with the paddle attachment of a stand mixer. I always do this by hand.
Add remaining cup of flour by quarter cupfuls until dough is no longer sticky. If you weighed your flour, you'll probably use it all. If you scooped, you might not.
Using dough hook attachment of a stand mixer, knead until dough is smooth and elastic.
Place dough in a greased bowl, cover, and allow it to rise in a warm place until doubled in bulk. With instant yeast, this takes about 45 minutes.
Punch down dough and turn out onto a lightly floured work surface. Roll HALF of the dough into an 8x12 inch rectangle. Brush with half of the softened butter. Mix together cinnamon and brown sugar, then sprinkle half of the mixture evenly over rectangle.
Starting at the short end, roll firmly into a log, pinch the ends closed, and tuck them underneath. Place in a greased and parchment lined 8x4 inch loaf pan. Repeat with remaining dough and filling ingredients.
Let rise in a warm place until doubled in bulk (30-45 minutes). Don't let it rise too long or it will deflate during baking.
Preheat oven to 350 degrees F (175 degrees C).
Brush the top of the loaves with melted butter, then bake in preheated oven until golden brown, and sound hollow when tapped, about 30 minutes.