Chill 4 martini glasses. Meanwhile, in a small saucepan, bring the sugar and ¼ cup water to a simmer. Remove from heat and let cool.
In a large pitcher, combine the sugar syrup with the vodka, juices, and Cointreau. Add 2 cups ice and stir until well chilled.
Strain the martinis into the chilled glasses; garnish each with a lime slice.