You need a regular Victoria Sandwich sponge cake recipe. You need to double the quantities to make four layers. My simple (doubled) recipe is 12 oz margarine, 12 oz caster sugar, 6 eggs and 12 oz self-raising flour.
Remove the four cakes from the sandwich tins and allow to cool. Then cut a circular hole in two of them. Place a whole layer at the bottom, then the two layers with missing centres, using some butter icing to stick the layers together. Fill the hole in the centre with small sweets - I used Smarties and gummy bears. You do need quite a lot to fill the hole - I used two bags of Smarties and one of gummy bears with a few left over for decoration. You could also use chocolate sweets like Maltesers and Minstrels, or perhaps Skittles. Then place the final layer on top and ice over the top and sides. The sweets packed inside will mean that the cake holds its shape.