Pour the milk into a pot and place it on the stovetop. Bring it up to a boil over medium heat. Stir continually to make sure that the milk does not burn on the bottom of the pan. When it comes to a boil, the milk will be 180°F. As soon as it boils, remove the pan from the heat.
Allow the milk to cool to approximately 110°F. A candy thermometer is useful to determine this temperature, but if you do not have a thermometer, you can use your CLEAN finger to judge the correct temperature. The milk will feel comfortably warm to the touch, like a warm bath. It will not be uncomfortable for you to put your finger into the milk.
While the milk is cooling, place a dish towel over a rimmed baking sheet to cover, and then place a large bowl onto the covered baking sheet. This larger bowl can be tempered glass, ceramic, earthenware, or stoneware. DO NOT use plastic.
Next, place a medium-sized bowl into the larger bowl. Make sure that the medium-sized bowl is larger enough to hold the 4 cups of milk. This medium-sized bowl can be tempered glass, ceramic, earthenware, or stoneware. DO NOT use plastic.
When the milk has cooled to the correct temperature, transfer the milk to the medium-sized bowl and add the yogurt to the milk. Stir the milk and yogurt together until the yogurt is completely incorporated.
Boil some water in a tea kettle. Once it comes to a boil, allow it to cool for a minute or so and then pour the hot water in between the two bowls. The water should come to right below the lip of the medium-sized bowl. (See video.)
If the bowls have lids, put the lid on the medium-sized bowl and then put the lid on the large bowl. If your bowls do not have lids, you can cover them with plastic wrap.
Cover the bowls with two dish towels or one large towel and let them stand in a warm place, undisturbed. An oven that has been turned OFF, but has the light on or a pilot light, is an ideal place. Other acceptable places include the top of a refrigerator or a pantry.
After 4 hours, check the yogurt. It should be set firm. If not, re-cover it and let it continue to culture for another hour. The longer you allow it to culture, the firmer it will become, but it will also become more tangy or tart tasting. Also, keep in mind, when you refrigerate the yogurt, it will also become slightly thicker without the addition of tanginess. This is important to note if you are planning on feeding the yogurt to children (or adults for that matter!) who do not like a particularly tart tasting yogurt.
When the yogurt has reached the consistency and taste that you like, refrigerate it. It will stay fresh for up to 1 week, refrigerated.