Boil 4 cups strained guava juice in an 8 quart saucepan for 10 minutes before adding 4 cups of sugar. Cook rapidly to 222 degrees or until mass of 3 or 4 drops run together and "sheet" off of the spoon. When using a jelly or candy thermometer, stir the jelly just before reading the thermometer in the center of the sauce pan. As soon as the 220 degree mark is reached, remove the jelly from the stove and pour into hot, sterilized jelly glasses. Allow jelly to cool for 1 to 2 minutes, then skim. Seal with paraffin.