In this article we sift through the evidence and predict what new features and technologies the Series 4 will offer (such as solid-state, haptic buttons and a blood-pressure-measuring band), as well as discussing what we would like to see next.
For information on the current range, take a look at our Apple Watch buying guide, as well as our roundup of the best Apple Watch deals.
As expected, Apple didn't mention the Series 4 at WWDC 2018 on 4 June. But when will we hear about the next update?
Three generations in, we're starting to get a feel for Apple's smartwatch launch schedule, which was irregular early on but appears to be settling down into an annual event.
These are onsale launch dates; the first model in particular was announced a long time ahead of this.
Apple Watch (first-gen): 24 Apr 2015
Apple Watch Series 1 & Series 2: 16 Sept 2016
Apple Watch Series 3: 22 Sept 2017
Apple Watch Series 4: Sept 2018?
Our money is on an update in autumn 2018, and most likely in September alongside new iPhones. This is backed up by a March report from analyst Ming-Chi Kuo, who points to an autumn 2018 update.
In June 2018 Kuo reiterated his expectation that a new Watch will arrive in the second half of the year, also claiming that this new Watch will feature a larger display (viaMacrumours).
There has been a separate rumour that Apple could release a between-full-updates 'S' edition of the Apple Watch: presumably a Series 3S, which would add extra storage options the Series 3 and come out before the Series 4. But until we see concrete evidence, we're not convinced by this.
Apple has kept the same design for three generations in a row (aside from some minor cosmetic tweaks, such as a new material and colour options, and a red spot on the cellular model). Having added a significant new feature in 2017 - cellular connectivity - we think the company will focus instead on an all-new design in 2018.
The longest Apple has stuck to the same design in its iPhone line-up, as a comparison, is three generations, from the iPhone 6 (via the 6s) to the iPhone 7 - the next update featured a major redesign for the iPhone X. Apple may feel that the watch sector is more aesthetically conservative than the phone one, but it would still be a surprise to us if the company broke its record and had four essentially identical smartwatches on the spin.
This is the most plausible significant design change, supported by no less than Ming-Chi Kuo, the popular and generally reliable analyst. In a report published back in March 2018, he predicted that the Series 4 would have 15 percent more screen space than the Series 3.
It's not clear how this would affect the overall design - namely, whether the chassis would get bigger too, or if Apple would shrink the bezels around the edge (we hope for the latter). But Kuo also predicts a larger battery capacity, which is usually linked to a larger device body.
The analyst thinks it's going to be a sharp design, however: he forecasts that a "more trendy form factor" will lead to improved sales.
In June 2018, Ming-Chi Kuo (now at TF International Securities) reiterated his belief that the new watch, when it launches, will sport a bigger display.
Haptic, solid-state buttons
Apple is working on an Apple Watch where the hardware buttons are replaced by touch-sensitive, solid-state haptic buttons, according to Fast Company, citing "a source with direct knowledge of Apple's plans".
Like the Home button on the iPhone 7 and 8, this button would not click in and out physically; rather, it would sense the proximity of the fingertip digitally and then simulate a click using haptic vibration. It follows that the button would nit be usable when the device is completely powered down, but by removing a moving part - a potential failure point, in other words - it could improve reliability, while also enabling users to digitally adjust the buttons' sensitivity.
The site adds that in the longer term Apple will release a watch with no buttons at all.
A recurrent rumour suggests that Apple will give the watch a round face, much like a traditional analogue wristwatch. To be honest we've never much liked the way this approach affects the interface on Android Wear watches (text gets squeezed at top and bottom, and usually ends up unattractively centre-justified), but patent activity suggests that Apple is at least considering the idea - or has previously thought about taking the circular route.
Patent 9,940,866, for an "electronic device having a display with curved edges" (as spotted by Apple World), argues that "circular displays or other displays with curved edges" would be a more efficient use of space in terms of pixel arrays.