Inspect two 5-ounce jars for cracks and immerse in simmering water until marmalade is ready. Wash lids and rings in warm soapy water.

Wash and dry grapefruits. Zest 2 grapefruits into ribbons and set aside. Use a knife to peel thin strips from the other 2 grapefruits.

Remove all white parts from grapefruits. Cut into wheels and remove seeds.

Place fruit and zest in a saucepan with sugar. Heat to a boil, stirring constantly. Smash fruit until liquefied. Reduce heat and cook for 10 minutes, removing froth as needed.

Add reserved zest ribbons to saucepan. Cook 5 minutes, test consistency using freezer method, and adjust cooking time.

Fill sterilized jars with marmalade, leaving 1/4 inch headspace, and remove air bubbles. Wipe rims clean and secure lids.