The RX10 IV, as the name suggests, is the fourth in Sony's series of 1"-type sensor, long zoom compacts. The Mark IV is the first to offer phase detection autofocus alongside a series of changes designed to boost the speed and capability of the camera, for both stills and video shooting.
Sony is adamant that the camera is much more than an RX10 III with an RX100 V sensor in it. Let's take a look at what the latest version brings.

One of the biggest changes in the Mark IV is the addition of on-sensor phase detection autofocus. There are a total of 315 phase-detect points, which cover 65% of the total sensor area. This is a significant update as it should eliminate the RX10 III's need to hunt for focus, which was a particular problem at the long end of the zoom.
In addition, we're told the camera has "exactly the same" processor as used in the company's flagship sports camera: the a9. This means the RX10 IV has the same autofocus algorithms for subject tracking and the improved Eye AF mode we saw on the a9.

The RX10 IV also becomes the first camera in the RX series to gain a touchscreen. This can be used for tap-to-focus in both stills and video mode. In video mode it is designed to offer a smooth focus transition between subjects which, combined with on-sensor PDAF, should make it relatively easy to shoot good-looking video without having to worry about manual focus.
The screen can also act as an AF touchpad when the camera is held to your eye, with the option of limiting the active area of the screen to one of nine regions of the rear panel, including the top (or bottom) right or left quadrants. There's also a choice as to whether the AF movement is absolute (pressing on the left of the screen places the AF point on the left of the image) or relative (swiping left anywhere on the screen moves the AF point left from its current position), as different photographers prefer different methods. These are all welcome improvements over previous touchscreen implementations from Sony.
Continuous shooting

Continuous shooting speeds have been dramatically improved since the RX10 III, with the max frame rate increase from 14 to 24 fps, with continuous AF. The buffer is substantial, to say the least, topping out at 112 Raw and 249 Fine JPEGs.
If that's too fast for you, middle (10 fps) and low (3.5 fps) options are also available.
Speaking of (very) quick, the camera's electronic shutter allows for bullet-stopping 1/32000 sec shutter speeds. The RX10 IV uses the e-shutter in order to shoot at 24 fps, by the way.
4K and proxy shooting

The RX10 IV can shoot 4K video from the full width of its sensor, which is rendered and downscaled to give very detailed, "oversampled" footage. This can be shot at 30, 25 or 24p in either 100Mbps or 60Mbps using the XAVC S codec. Dropping down to Full HD (1920 x 1080) you'll find 120p, 60p, 30p and 24p frame rates. If you're so inclined, a 60i option is available if you switch to AVCHD. (The PAL equivalents for these are also available, of course.)
As mentioned earlier, the new touchscreen display allows for tap focusing. You can use this to "rack focus" with zero effort, and there are three transition speeds to choose from. Unfortunately, 'Spot Focus' continues to confuse, and there's still no easy way to 'tap to track' a subject, as all Lock-on AF options are greyed out in 4K video mode. It is available in 1080p video, but only via the rather clunky (and old) 'Center Lock-on AF' method.
The Mark IV also gains a 'Proxy' shooting mode, where it captures a 720p stream of video alongside the main 4K stream, meaning you can edit using the proxies and then apply the edits to the full-res footage at the end of the process. This greatly speeds up the workflow, especially when using slower computers.
High frame rate shooting

In addition to 4K capture, the RX10 IV is able to shoot 1080 at up to 120p, which can either be saved as 100Mbps or 60Mbps clips or slowed down, in-camera, to 60, 30 or 24p.
The camera has the ability to capture at 240, 480 or 960 fps, with footage taken from increasingly low-res crops from the sensor (250, 500 or 1000 fps in PAL modes), which can then be output as 60, 30 or 24p super slow-mo footage (50 or 25p in PAL).
Other improvements

The RX10's focus peaking has also been improved, with three intensity settings designed to make the peaking easier to see and distinguish between, as you shoot.
A new focus limiter button, found on the left side of the camera, lets you choose between the whole focus range or 3m to infinity. Sony has also added a new "AF-A" mode, which will choose between AF-S and AF-C depending on subject movement.
Fans of back-button focus will be pleased to hear that you can now activate autofocus with any of the custom buttons (we figure most folks will use the AE-lock button).
Another new feature is Bluetooth connectivity, which can be used to share location data with the camera. We'll see what else it can do when we spend more time with the camera.
Something that's a slight step backward is battery life, which drops from 420 to 400 shots per charge (CIPA standard).

The Mark IV uses the same 24-600mm equivalent, F2.4-4 zoom lens as its predecessor. As, no doubt, people will be highlighting in the comments, this is an equivalent aperture range of F6.5-10.9, which is not significantly slower than an F4.5-6.3 tele zoom on an APS-C camera. Even with that, the lens quality is superb, especially considering its long reach.
As one would expect, the lens is stabilized, and Sony claims 4.5 stops of shake reduction using CIPA's testing methods. The company says that it has improved the IS system at the long end of the focal range, which should framing subjects easier.
Those who were hoping for the return of an ND filter (found on the RX10 II) will be sorely disappointed, as the RX10 IV lacks one as well. The lens is threaded for 72mm filters, however.

$1700 is a lot of money, but Sony believes the combination of capabilities: high speed shooting, autofocus performance and 4K video capture, together with a 24-600mm equiv. zoom, is what makes the Mark IV a compelling offering.
What do you think? Share your thoughts in the comments below.