Handling
The Audi A4 and the BMW have about similar tires.
With their comparable turning radii, the BMW 3 Series and the A4 most definitely handle roughly the same into and out of tight spots.
Drivetrain
The Audi's engine isn't much bigger than the BMW 3 Series's.
Utility
There's no clear winner between the A4 and the BMW 3 Series when it comes to transporting cargo.
The Audi A4 and the 3 Series can accommodate the same number of passengers.
Convenience
You'll very likely take the Audi for a fill-up approximately as often as the 3 Series.
You most definitely won't have to visit your friendly gas station more often with the Audi A4 than with the BMW 3 Series, because both offer similar mileage ranges.
Comfort
Taller drivers will significantly enjoy the extra head room in the Audi A4's front cabin compared to the BMW.
While the rear of the BMW offers a bit more head- and leg-room than the Audi A4, there honestly isn't much of a difference.
Dimensions
The Audi A4 is somewhat more ponderous than the BMW 3 Series.
The BMW 3 Series's tank is about the same size as the Audi's, though the A4's is a little more capacious.
The BMW and the Audi A4 compete for the same parking spaces.
Cost
The destination charge is a standard charge for transporting the vehicle from its point of origin to the dealer. It costs about the same to get the BMW 3 Series to the dealership as the Audi A4.
The A4 pales in comparison to the BMW 3 Series with respect to gas mileage.
The Audi A4 won't tax your gasoline budget significantly more than the BMW 3 Series will.
In terms of MSRP, the BMW 3 Series costs roughly as much as the A4 (though the A4 is a tad cheaper).
Overview
The BMW 3 Series and the Audi have the same basic after-sale protection.