Since the first Surface RT released in 2012, Microsoft's ambition has been to build a tablet that can replace your laptop. This was always a dangerous way to position the systems, because it drew comparisons with real laptops. Microsoft didn't even shy away from those comparisons, explicitly comparing the Surface Pro 3 to the contemporaneous MacBook Air.

In terms of size, weight, price, and performance, the comparison made some sense, but it was double-edged. While it highlighted the Surface Pro 3's undoubted advantages—better screen, touch and stylus input, tablet form factor—it also made clear the Surface Pro 3's big deficit relative to the laptop: it's not actually very good on your lap.

This is either crippling or irrelevant. There are plenty of laptop users—and I used to be one—who need a laptop because they want a machine that is portable, easily moved from desktop to coffee shop to kitchen table. In this role, the Surface Pro 3 had few real issues. Its footprint was a little larger than a similarly sized laptop due to the kickstand, and its touchpad remained a weak spot. But as a package it worked well: the Surface Pro 3 was a flexible, capable machine that was effective as a tablet but offered much of the productivity of a laptop.

Then there are laptop users—and I am one today—for whom the word "laptop" has a very literal significance. I use laptops on my lap, on planes, at conferences, in bed, and the Surface Pro 3 wasn't very good in this respect. The lack of a stiff hinge to maintain a chosen angle between the screen and the keyboard meant that without a nice even surface to rest on, the Surface Pro 3 was not at all convenient to use.

When Surface Pro 3 worked, however, it worked very well. It was an improvement on its predecessors, as the Surface Pro and Surface Pro 2 didn't really convince as laptops thanks in particular to their 10.6-inch screens. With the Surface Pro 3, Microsoft made its tablet a whole lot more laptop-like. In particular, it bumped up the screen size and resolution to 12 inches and 2160×1440. This in turn made the Type Cover keyboards a little larger and provided an all-around improvement in ergonomics and usability: it made the device comfortable to use at laptop-viewing distances. The Surface Pro 3 also included a variable position kickstand, providing a level of adjustability that was comparable to that of a laptop. Where the first two iterations of the Surface Pro were really kind of flawed, the third felt like it was hitting its stride. If you could live with its not quite a laptop concept, it was a fine machine.

Specs at a glance: Microsoft Surface Pro 4 Base Best As reviewed Screen 2736×1824 12.3" (267 PPI), 10-point capacitive PixelSense touchscreen OS Windows 10 Pro CPU Intel 6th generation Core m3 Intel 6th generation Core i7 Intel 6th generation Core i5 RAM 4GB 16GB 8GB GPU Intel HD Graphics 515 Intel Iris Graphics Intel HD Graphics 520 SSD 128GB 1TB 256GB Networking 802.11ac/a/b/g/n with 2x2 MIMO antennas, Bluetooth 4.0 Ports Mini-DisplayPort, headphones, micro-SDXC, USB 3.0, Cover port Cameras Rear: 8MP autofocus, 1080p video

Front: 5MP, 1080p video, infrared facial recognition Size 11.50×7.93×0.33" (292×201.3×8.5 mm) Weight 1.69 lb (0.766 kg) 1.73 lb (0.786 kg) Battery 42 Wh Warranty 1 year Price $899 $2,699 $1,299 Sensors Ambient light sensor, accelerometer, gyroscope, magnetometer Other features 36 W charger with 5 W USB charging port, Surface Pen, TPM 2.0

Refinement, not reinvention

Surface Pro 4 takes the concept and refines it. The screen is bigger; the processor is faster; the keyboard cover is better; the pen is more accurate; the touchpad is transformed. The hardware is also more diverse with a wider range of performance options available.

It makes sense to start with the screen. Enlarging the screen was arguably instrumental in enabling the Surface Pro 3 to truly carve out its niche, and Microsoft has put a bigger, even higher resolution screen into the Surface Pro 4. What was a 12-inch, 2160×1440 unit is now a 12.3-inch, 2736×1824 display. This is 267 pixels per inch, up from 216.

Microsoft is calling the screen technology "PixelSense," which is amusing from a historical perspective. This is the second brand name that Surface has stolen. Microsoft's first Surface was a touch-screen table running Windows Vista. It lost its name to the tablets, with the second generation instead being called the Samsung SUR40 with PixelSense. That PixelSense name is now being used for Surface Pro 4's screen (replacing the previous ClearType branding), with Microsoft explaining that the screen is no longer just an output device; it's now an I/O device.

There are a few aspects to the PixelSense identity. The screens are oxide LCDs, using a metal oxide semiconductor instead of the more traditional silicon to reduce thickness and improve response times. The screens are also optically bonded to the touch sensors for reduced thickness and increased rigidity. The contrast ratio is high (1300:1) and the color gamut is full sRGB with screens calibrated at the factory. The screen is thinner than in the Surface Pro 3 with an even smaller distance between the screen's surface and the display beneath. This is particularly important when using the stylus, since it means that digital ink has less separation from the pen tip (which makes it more precise and natural to use).

The touch/pen controller is also new. Microsoft bought touch controller firm N-Trig earlier this year, and the Israeli firm is now working with the rest of Microsoft's hardware division to produce touch controllers not only for PCs and laptops but also phones.

The controller used in the Surface Pro 4 is called G5. Microsoft claims a signal-to-noise ratio about 30 dB better than the previous controller, whatever that means in practice, and more interestingly, the controller apparently uses GPU-based computation for processing its input data instead of the application-specific integrated circuit used in prior controllers. This enables both more complex algorithms to be used—better palm rejection, for example—and lower latency sensing. This is particularly important for pen applications, to ensure that ink appears quickly for an authentic and accurate experience.

The pen has also been upgraded; it now has 1,024 pressure levels (up from 256), and it's much more intuitive: the cap now functions as an eraser. The cap is still a Bluetooth button that can start up OneNote with a click. You can also take a screenshot with a double click and start Cortana with a long press.

Does this all make a difference? A little. The Surface line has always had good screens, and Surface Pro 4 has a good screen. It's big; it's bright; colors look good, and the resolution ensures that everything looks sharp. Side-by-side with a Surface Pro 3, it's definitely a better screen. But the other "PixelSense" aspects are harder to discern. I'm not an artist and my handwriting is abysmal, so I'm not a huge user of the pen. I think that Surface Pro 4 feels a little... crisper, with inking being a little more immediate, but it's a marginal improvement.

Then again, this is not a big surprise. Once you reach certain thresholds—"retina" resolution, a thin, bonded screen, IPS viewing angles—incremental improvements are as much as we can hope for. To the extent that the Surface Pro 4's screen doesn't knock my socks off, it's only because its predecessor was good, too. Put it alongside a MacBook Air, for example, and the comparison would be very different.