In the other bowl measure your 4 cups melted fat. It can be a combination of melted tallow and other oils. Olive, Almond, Apricot or Coconut work well. I add 1/2 to 1 cup of these oils to the tallow. 

Set bowls aside to cool. The lye must cool to 90 to 95 degrees F while the fat should be a bit warmer 120 to 130 degrees F. This cooling stage takes awhile. While waiting for the cooling stage grease your molds with petroleum jelly. Find a place where you can leave them undisturbed for several hours while the soap hardens. 

When the lye and oils have cooled slowly pour the lye into the fat, stirring constantly. When the lye mixes with the fat, the chemical reaction known as saponification occurs. Stir thoroughly so that no free alkali or fat remain.  As you stir the mixture, it will become cloudy, then somewhat grainy. As it approaches the pouring stage it will thicken and look like cooked pudding. Take your spoon and dribble some of the soap back on top of the mixture. If it leaves a ribbon that doesn't merge with the rest of the soap it is ready to pour. If you want to add scent, do it now. 

Pour the mixture into your molds. Smooth the tops of the molds with your rubber spatula. Work fast as the soap sets up quick. Let the soap set up for a couple of hours, then cut it into bars. 

After 24 hours, remove from the molds and let age for 3 weeks.