Cut 3 of the melons in half.
Cut a thin slice from the bottom of each half so that it will stand upright.
Remove and discard seeds and membrane; scoop out the fruit, reserving shells.
Whirl fruit in a blender to puree; measure.
If less than 4 cups, cut fruit from remaining cantaloupe and puree enough to make up the difference.
Add sugar, lemon juice and salt, stirring until sugar is dissolved.
Freeze in freezer trays until mushy.
Beat in a chilled bowl until smooth but not melted.
Return to trays and freeze until firm.
If desired, cut the shell edges into a zigzag design; chill well.
Just before serving, fill shells with sorbet and garnish with mint leaves.