I don't know what recipe you are using, but 12" is a very big sponge cake. A Victoria will be unlikely to rise much if you are aiming for a 4" deep cake because the weight of mix will compress the bottom. Also there is the problem that the width of the cake means that the inside will take ages to heat through and cook. You need something with more structure to it like a Madeira cake.
This recipe was sent in by a customer and it seems to have worked for many people I have passed it on to:
Madeira Cake - 12 Square (with thanks to Jen Aldred) (Cooking time 65 mins)
Grease and line the pan (Triple lining is suggested to prevent scorching).
Take a LARGE mixing bowl, cream the butter and sugar. Then add the eggs, one at a time to prevent curdling (you may need to add a little flour from time to time). Once the eggs are mixed in, fold in the balance of the flour.
Cook for 30 minutes at 180 deg. then cover with baking parchment for another 20 minutes at the same temperature. For the final 15 minutes, keep covered but reduce the temperature to 170 deg. (Gas 3).
This should produce a cake that has a close, even texture, is completely cooked and moist.