It's not easy to beat BMW in their own game but it can be done. Infiniti has done it with G35 and M45 (according to US magazines) and Audi has done it too with A8.

It's true that next A4 will be larger than current model, that will happen because most buyers care very much about interior space, but not as large as Passat. Audi will have to keep the weight as low as possible while making the chassis as rigid as possible. That will be posible with the combination of extended use of ultra strong steel and advanced welding techniques.