Heat 4L of milk in a double boiler to 180 F then turn off and let sit for 10 min.
Cool in ice bath until it reaches 110 degrees.
Remove a cup of the milk and place in a small bowl. Add 4 Tablespoons of yogurt and gently stir until combined. Add the diluted yogurt to the main batch of milk and stir gently.
Dilute 4 drops of rennet into a 1/4 cup of filtered water. Add the diluted rennet to the pot of milk and stir VERY gently to throughly combine.
Let sit 12 hours at 105 – 110 . Strain for 30 min. Whip.
To sweeten the skyr, add a 1/4 cup of sugar to the whipped skyr and the zest of one lemon.