The Sony Cyber-shot RX100 IV (M4) is less of an clear upgraade over its predecessor the RX100 III (M3) than a forked alternative; for a variety of reasons, I'd classify it as an advanced compact for video enthusiasts rather than a straight-up advanced compact, at least until the price drops a bit. Though it's more expensive and adds a plethora of video capabilities, it's pretty much delivers the same photo quality in the same body.
Officially priced at $950 (£920, AU$1,400), the M4 goes up against the less expensive Panasonic LX100 ($800, £700, AU$1,000), which also shoots 4K, though doesn't offer quite the breadth of video-related features that the M4 does.
Despite using a completely new -- but same size and resolution -- sensor, the images from the M4 look quite similar to those from the M3. The new stacked CMOS sensor, branded Exmor RS, gives the photosites their own layer while the circuits occupy the layer beneath. But the modification doesn't seem to have had a big impact on photo quality. It displays slightly better noise performance above ISO 800, with more detail preserved in dark areas, but nothing jumps out as "wow" better.
The photo quality is still quite good, sharp with decent color and tonal range, though I don't think surpasses the lower-resolution but larger-sensor LX100. There isn't a lot of detail to recover in the highlights.
The video quality is very good, as sharp as you'd expect from 4K. While I prefer the more accurate rendering the LX100 produces with little fiddling, the RX100 IV has tons of controls to get the results you want along with a quieter lens and adjustment dial.
The new sensor uses high-speed processing and additional memory below the circuitry for passing the image data along to the next stop in the process a lot faster. Sony also claims that this speed improvement defeats the jello and leaning verticals that you see during fast horizontal movement. There's still quite a bit of wobble that comes from camera shake when the image stabilizer is in Standard mode -- that's the best you can get when shooting 4K.
The RX100 M4 really does improve over the M3 in most -- though not all -- aspects of performance, and ranks as one of the fastest compacts we've seen. Like other cameras with zoom lenses, and unchanged from the M3, it's relatively slow to startup since you've got to wait for the lens to extend. Once extended, though, it's consistently fast: about 0.2-0.3 second to focus and shoot, regardless of lighting conditions, and 0.2 second for two sequential shots, in both raw and JPEG. With flash enabled that time increases to about 2.3 seconds, also unchanged from the M3.
The biggest deal for the M4 is its much hyped continuous-shooting performance. On one hand, its tested rate of 5.7 frames per second for JPEGs and 5fps for raw, both for at least 30 shots and with autofocus, is very good for this class.
But the camera's actual high-speed shooting is far more complex than a spec conveys. With the mechanical shutter enabled and with autofocus, it drops to 2.4fps; the faster rates require the electronic shutter. (One of the differences between the M4 and its predecessors is the ability to manually select mechanical or electronic shutter, though there's still an auto option.)
In the limited number of situations where you can forgo autofocus and autoexposure, usually when people are standing in one place and moving just parts of their bodies, Sony's Speed Priority Continuous can do 10fps with the mechanical shutter and 16fps with electronic. There's quite a wait for the camera to process the photos before you can get into playback or the menus, though it doesn't stop you from continuing to shoot.
The autofocus works pretty well, although when you use continuous AF and see the way it pulses for every exposure you can tell why it the burst with AF is so much slower. I didn't have much luck with the object-tracking autofocus either, as (like most) it's easily distracted by objects which cross your subject's path. The single-shot autofocus is pretty good, but the Wide-Area AF -- Sony's version of leaving the choice of AF subject to the camera -- performs about as well as other cameras'. That is, when zoomed out it selects everything in the scene and when zoomed in it selects the closest objects.