Preheat the oven to 350°F/180°C/gas mark 6. Oil an 8-inch (20cm) cake pan and set aside.
In a small bowl, mix the flour, Chai spice mix, baking powder, and salt. Set aside.
In another bowl, mix the oil and sugar until the sugar dissolves completely. Add the milk and the eggs. Whisk together with a fork. Set aside.
Core, peel and grate 2 of the apples and add to the bowl of wet ingredients.
Add the wet mixture to the dry mixture. Fold them together with a spatula. Be careful not to overwork the batter. If you are using nuts, add them now.
Gently spoon batter into the prepared cake pan.
Core and cut the remaining 2 apples into thin slices and gently push them into the batter in concentric circles starting at the outside and moving into the center.
Bake for 35-40 minutes, or until a toothpick comes out clean.
Leave to cool completely on a wire rack before cutting.