What is the difference between Audi A3 and A4's engines?

I am currently considering purchasing an Audi A3 or A4 model, but I am confused about the engine specifications. Could someone please explain the main differences between these two models' engines?

The Audi A3 has a smaller, more fuel-efficient engine compared to the A4. This results in a lower horsepower output but better fuel economy.

The A4 offers a more powerful engine with higher horsepower and torque compared to the A3. It also has a smoother and more refined driving experience.