Adapted from The Kitchn
Activate the yeast by putting it in the lukewarm water with a pinch of sugar and let sit for a few minutes.
Mix the 4 cups of the flour, sugar, and salt in the bowl of a standing mixer (or a very large bowl if you're mixing by hand).
To the dry mixture add eggs, yolk, yeast, and melted butter and then mix to form a shaggy dough.
With a dough hook attachment, knead the dough on low speed for 6-8 minutes. (Or on a floured surface knead by hand for about 10 minutes.) You can add up to 1/2 cup flour to create the consistency you like. The dough has finished kneading when it is soft, smooth, and holds a ball-shape.
Let the dough rise in an oiled bowls until doubled, about 1-2 hours.
Separate the dough in to 6 even pieces and roll into ropes about 16 inches long.
Braid the dough. Squeeze all six ropes them together at the top then starting from the right most rope braid as follow: over two ropes, under one rope, then over two ropes again (the rope will end up on the far left). Continue with the next right most rope until finished braiding then pinch all the end ropes together and tuck under to clean up the shape. Push together bread for a plumper loaf.
Let the braided challah rise about an hour until the loaf looks puffy again.
Brush the challah with egg white and top with sesame seed or poppy seed (or anything really) if desired) then bake the challah at 350°F on a baking sheet for 30-35 minutes. Cool slightly before eating or slicing