Wash and dry the lemon, lime, orange and grapefruit. Cut the fruit in half and cut each half into thin slices. Divide the fruit slices among 6 mason jars, then add 2 ounces of vodka and 1 teaspoon of sugar to each. Screw the lids of the jars on tightly, then shake each jar.
When ready to serve, top each with 6 ounces of citrus soda.