Fill 4 tall glasses three-quarters of the way with ice. Pour 1/2 teaspoon grenadine into each glass. Combine 1 cup tequila, 1 cup grapefruit juice, 1/4 cup fresh lime juice, 2 teaspoons superfine sugar and a pinch of salt in a cocktail shaker. Shake well, then pour into the glasses. Top each drink with 1/4 cup grapefruit soda.