As of right now there’s no aftermarket subwoofers fitted for the 4g that I’ve seen. The Rockville RW10CA might be small enough but the space behind the rear seats is less than the previous generation so it pretty much leaves you room to only do a system under the seats. Best option I’ve seen is...
Front: 3 way component system including a 5 ohm 7” x 8” (weird I know) speaker in the front doors, tweeter in the A-pillar, and 2.5” speakers in the dash
Rear: not completely sure yet but should be a normal 2 way 5.5” coaxial speaker
Subwoofer: tiny ~8” subwoofer behind the rear driver seat...