Bring the pomegranate juice just barely to a boil in a saucepan over medium-high heat.
Reduce to a simmer, and simmer for 1.5-2 hours**, until the juice has reduced down to 1-1.5 cups. Give it a stir every 20 minutes for the first hour, then every 10-15 minutes. Keep your eyes on it so it doesn't burn! It won't be super thick yet, but will coat the back of a spoon.
Let it cool to room temperature. It will be thicker now.
Store in an airtight container in the refrigerator for up to 6 months.
Video
Notes
* If using just 4 cups, look to yield 2/3-3/4 cups of pomegranate molasses. I use the brand POM Wonderful, which comes in a 6-cup bottle, 48-fl. oz.** It needs to be at just a simmer. If you're boiling it too vigorously, it will quickly evaporate and burn. Keep your eyes on this one, and don't boil it! Boiling it will cook the juice and not be as tasty.