What are the key differences between the Audi A3 and A4 models?

I am considering purchasing either an Audi A3 or A4 and would like to know the main differences between the two models.

The main difference between the Audi A3 and A4 is their size and price range. The A3 is smaller and more affordable, while the A4 is larger and more expensive.

Another key difference is in their powertrain options. The A3 offers smaller, more efficient engines, while the A4 offers larger, more powerful engines.