Let's get it out of the way: no, the 4-Series doesn't have a very useful or comfortable back seat, not if you're in your second decade on the planet or in anything beyond a size medium. If you're lucky enough to be riding up front--or driving--it's an entirely different story.
The 4-Series might have a lot in common with the 3-Series sedan, in terms of engines and powertrains transmissions, but it doesn't use its identical wheelbase or overall length to impart as much interior space. It's the same 110.6-inch wheelbase, and overall length of 182.5 inches, but the 4-Series also has a roofline two inches lower, while it's also a half-inch wider than the 3-Series sedan.
You sit quite low in the 4-Series, and grip a steering wheel that's as thick as any SUV's wheel; you'll learn to love the seatbelt presenter because reaching for the belt every time would lead to rotator-cuff problems down the road. Even the front base seats are firm and comfortable--the kind we'd approve of for daily use as desk chairs--but they don't have as many adjustments or the very firm bolstering of the excellent sport seats offered on the M Sport package. Those seats grip in all the right places for the kind of sporty driving the 4-Series encourages; heating and ventilation are a part of the deal.
Though it's three inches wider across the rear axle, the 4-Series still doesn't net out with much adult-sized space in the back seat, and in a tight garage its long doors don't make entry or exit very easy. It's be hard to justify the 4-Series as a four-seater on a regular basis, but the rear buckets are fine for shorter people on shorter trips.
Trunk space is almost the size of a mid-size sedan, though, and storage in the cabin is a brighter spot than ever, with real cupholders ahead of the joystick-style shifter and a decently-sized glove box, even some bottle holders molded into the door panels.
The base models, with basic black interiors, can seem drab and dark--and they amplify the 4-Series' sweeping dash curves in an unflattering way. Go all in with the luxury touches, though, and the coupe's interior gets the sophisticated allure of the bigger 6-Series, with handsome leather and contrasting stitching, some daring colors and some very pretty wood and metal finishes. BMW's wide, beautiful LCD screen displays crisp maps and iDrive functions, but it's also propped up the dash like a digital picture frame, something it has in common with the Mercedes CLA. Audi's A3 has a better idea: make a slide-away screen standard.