Back when the Nikon 1 range was first launched in 2011, there were only two lines: the J range and the V range. Initially, it was the V range which was the more advanced and had better features than the more entry-level J range.
Now however, there are more lines in the 1 line-up, and while the V remains top dog, there is now the S range which makes up the entry-level end of the spectrum, placing the J range in the middle of the line-up.
Whereas before there was quite a big distinction between J and V cameras, the J4 is very similar to theV3 in many ways. It features the same 18.4 million pixel CX format (that's one inch) CMOS sensor as the V3, as well as the same Hybrid Autofocusing system - that is 171 contrast detect points and 105 phase detection points. There's also the same Expeed 4a processing engine.
The one-inch sensor is significantly smaller than most other compact system cameras, including Micro Four Thirds offerings from Olympus and Panasonic and APS-C sized devices from Sony, Fujifilm and Samsung.
Recently however, Samsung has also introduced the NX Mini which also has a one-inch type sensor. One of the plus sides of having a small sensor is that the overall system (including the lenses) is generally smaller than its competitors.
Although there aren't quite so many lenses available for the Nikon range as there are for Olympus, Panasonic and Sony cameras, the range is growing. You can also attach existing F mount lenses using an adapter - but you should bear in mind that anything attached has a 2.7x focal length conversion - so a standard 50mm lens has a focal length equivalent of 135mm.
The J4 comes with a 10-30mm f/3.5-5.6 kit lens, which offers a 35mm equivalent of roughly 27-80mm. Unlike previous generations of this lens, the new 10-30mm is a retractable power zoom lens, which means that not only does it automatically extend and retract on power on and off, but you also use a switch to zoom the lens in and out, rather than by twisting the lens barrel itself.
Nikon seems to have decided that Nikon 1 users aren't too bothered by viewfinders as it has ditched them on both the V3 and the J4, however while the V3 has a hotshoe for attaching such accessories, there's no such joy on the J4. So if you're the kind of person who likes to compose in a traditional way, you'll be out of luck here.
On the plus side, the J3 is equipped with a three-inch 1037k TFT LCD screen, which is touch sensitive. This means you can use the screen to set autofocus point and access some of the menu settings. Unlike the V3's screen, the J4 is fixed (and doesn't tilt or articulate).
The Nikon 1 range of cameras have an exciting set of specifications, the likes of which aren't really found on anything else. For instance, Best Moment Capture automatically shoots a series of pictures from which you can choose the best image. It's also capable of incredibly fast shooting, with up to 60fps using a fixed autofocus point, or 20fps with continuous autofocus - that's hugely faster than most DSLRs can manage.
Along with automatic controls and the various different shooting modes that the J4 offers, you can also shoot in fully manual or semi-automatic modes (aperture priority and shutter priority). A new introduction for the J4, like on the V3, is Creative Palette mode. This allows you to preview different filter effects before you take the shot.