Place 3 cups of the water in a pot along with the sugar and bring to a boil. Boil until the sugar is dissolved, about 1 minute. Remove the pot from the heat. Add the tea bags and steep 5 minutes. Place the raspberries and remaining 1 cup of water in another pot. Bring to a boil and boil 1 minute. Strain the liquid through a fine sieve into the pot with the tea mixture. Press the raspberries with the back of a ladle to extract as much juice out of the berries as you can. Cool to room temperature. Transfer to a serving jug and store in the fridge until needed. To serve, place 4 or 5 ice cubes in a glass. Drop in a few whole raspberries; garnish the rim of the glass with a 1/2 lemon slice. Pour in the tea and serve.
Options: For added colour and flavour, drop a mint sprig into each glass before pouring in the iced tea. Instead of orange pekoe, try another tea, such as green or jasmine. Make blackberry iced tea by replacing the raspberries with an equal amount of blackberries.