Juice the lemons until you have one cup of fresh lemon juice.

In a small saucepan, combine one cup of sugar with one cup of water and heat gently, stirring until the sugar is fully dissolved.

In a large pitcher, combine the fresh lemon juice, simple syrup, and the remaining three cups of cold water, stirring well to combine.

Pour in the vodka and stir to mix evenly.

Taste the mixture and adjust sweetness or tartness as needed.

Refrigerate the hard lemonade for about 30 minutes to chill.

Fill glasses with ice cubes and pour the chilled hard lemonade over the ice.

Garnish each glass with a lemon slice.