How much white chocolate should I use in a white chocolate cake?
Looking at your recipe, there's not enough butter or eggs for the amount of flour and sugar. I'm saying this because I'm assuming the the oz measurements are weights. You could also increase the amount of white chocolate to 6 oz. 

Since white chocolate doesn't have much of a presence in a cake - the primary flavour is vanilla - you may want to make a good white or yellow cake and save the white chocolate for a spectacular icing.