The newest Apple Watch Series 4 is now available alongside new iPhones, and this year, it's the product line with a more dramatic story to tell. While the iPhone XS and XS Max are more subtly-upgraded "S"-year evolutions of 2017's iPhone X ($999 at Walmart), the Apple Watch Series 4 is the biggest year-over-year makeover Apple's wearable has seen in its three-plus-year history. 

New for 2018 is the Apple Watch's first big redesign ever: The screens are bigger, the body is thinner. And the watch's already-impressive heart and health tracking gets a truly groundbreaking addition: A built-in electrocardiogram (EKG or ECG), the first-ever to be integrated directly into a smartwatch. The feature is already FDA cleared in the US, where it is scheduled be activated by a software update later this year. 

The Apple Watch Series 4 start at $399, £399, AU$599 for the standard model, and $499, £499, AU$749 for the LTE model. Models climb up from there depending on size, material (stainless steel versus aluminum), band choices and additional styles (Nike+, Hermes).  

I got my hands on the Apple Watch Series 4 earlier this week, in the new stainless steel gold option. This will be an ongoing review looking at the watch and its features. I'm already liking the new, larger watch faces and their additional customization options, so let's start there.

Originally published Sept. 19, 4:40 p.m. PT. 

Updated Sept. 21, 2:50 p.m. PT with additional impressions on battery life.

First impressions: All-new design, larger screen

The Apple Watch S4 is the biggest design update since the watch launched in 2015. Apple followed the model of its recent iPhone designs here: The Series 4 cuts down the bezel surrounding the screen versus last year's Series 3, effectively squeezing larger screens into the same size chassis. It's still a squircle but the look is much more refined. It's thinner, too: Apple cut the depth of the Watch from 11.4mm to 10.7mm, so it's less bulbous on your wrist.

New sizes: The Apple Watch still comes in "small" and "large," but thanks to the economy of sizes from the redesign, the smallest new Apple Watch has a display that's close to the size of the previous large one. The new watch sizes are 40 and 44mm, vs. the previous 38 and 42mm.

New watch faces stand out: New watch faces come with the larger display, and many existing Apple Watch faces have been subtly upgraded to be bigger or otherwise take full advantage of the new real estate. Some add a lot of new complications, or bits of information. (Complications are basically at-a-glance pieces of watch information that double as app launchers.) The watch's richer complications show more info, now, and even some graphics. More shortcuts means launching apps faster -- or not having to launch them at all, because the info you need is already on the screen. The new S4-exclusive faces, called Infograph and Infograph Modular, are fantastic. But sometimes, they can feel like a bit too much. For a full analysis, read my watch face impressions.

These new watch faces are, in my opinion, the key to how the Series 4 can differentiate over older models. Right now, there aren't as many as I expected. Those that are here are already my most-used.

The rest of Apple's watch faces end up being pretty familiar. There are 25 of them, with a ton of customization and variation. But Apple still doesn't have a watch face store, meaning that you're limited to the ample but contained collection of faces Apple has on-watch. These can be customized, but the limits become clear fast. Apple's main competitors -- Fitbit, Samsung, Google's WearOS -- all allow third-party watch faces: When will Apple allow this to happen?

Other thoughts on the new design: The fuller display and its iPhone X-like curved edges is a big upgrade, and the slightly thinner design makes the older S3 watch look more square and bulbous. The digital crown on the side has subtle haptics now, so turning it produces realistic-feeling click mechanisms. It's satisfying, but not necessary. The side button below lies more flush now. More importantly, the onboard speaker is louder and clearer, making Siri and phone calls a lot more useful (especially in emergencies).

Good news on your old accessories: Bands fit the same as the older Apple Watches. I swapped my old ones in and traded bands to check. (Make sure you have the right size, though: 42 and 44mm sizes are interchangeable, and likewise 38 and 40mm.) The gold-colored steel Milanese band on my review model is a bit over the top for me.

Fitness promises some big boosts

The rear heart-rate sensor has been completely redesigned this time around, with a lot of heart rate potential. This Apple Watch promises more accurate heart-rate sensing, step tracking, fitness detection, and even fall detection, which will automatically call 911 as well as your loved ones if you fall while walking, or on a bike (testing on that to come).

And the wildest thing is an embedded EKG sensor, allowing FDA-cleared electrocardiograms. It will enable spot detection of more detailed heart rate and rhythm, and spot possible arrhythmia. That feature is not being unlocked until later this year, though. In the meantime, the Watch indicates drops in heart rate, in addition to the elevated heart-rate warnings available in earlier models. Even before the EKG launch, the Apple Watch is taking on the role of a true medical evaluation tool. Vanessa Hand Orellana and I will be looking at these new fitness features in greater depth in the upcoming days and weeks.

Read: The Apple Watch Series 4 takes heart monitoring to next level

Battery life: So far, not much has changed

The Apple Watch S4 doesn't promise better battery life, really, except during GPS-enabled workouts (six hours, which still is a lot less than dedicated GPS fitness watches, such as models from Garmin). We'll be testing workout battery life soon. But I've been wearing the Apple Watch Series 4 with cellular, paired to an iPhone X, and seeing how it does. More than a day later, it looks like the watch lasts for one solid day on a charge. I've been tracking my daily battery life levels on Twitter.

I charged the 44mm watch up and began a full day starting around 7:20am. I kept cellular on and Wi-Fi on, but the iPhone paired and nearby most of the time. Going to bed close to 1 am, battery life was around 24 percent.

That looks like it's around the same as the 2017 Series 3, as Apple claims, despite slimming down the size of this model and adding a larger display. I used to get a day and a half on the Apple Watch S3, on average. Over a full day, battery life will be fine. 

But the Series 4 is still a watch that needs daily charging, it seems. And I wish that weren't the case. Fitbit's watches last five to seven days. Other fitness smartwatches last at least two days. I'm still testing it, and will check out how it fares with cellular off, and on its own with cellular on (expect less battery life there).

Better battery life is a frontier that Apple hasn't cracked yet. Qualcomm is promising longer smartwatch battery life on Google WearOS watches, with lower-power modes measured in weeks, not days. If Apple has any desire to add sleep tracking to the Apple Watch's bag of health tricks, it's going to need to go down the same path for future models.

Should you upgrade?

I've just been using this watch for a few days, so I'm not close to making a buying recommendation yet. Here's what I'd say in the meantime:

Series 1/2/3 users should upgrade to WatchOS 5 first: The latest version of WatchOS became available earlier this week alongside iOS 12. It adds it a bunch of nice extras to those 2016 and 2017 watches, including downloadable podcasts, a walkie-talkie mode and more watch faces. Try that out first -- It could help pep up your older Apple Watch.

2015 Apple Watch owners have the most to gain: The original "Series Zero" Apple Watch can't be updated to WatchOS 5. A Series 4 will be a compelling upgrade for anyone who wants a bigger screen and more speed.

Series 3 owners will have the most difficult decision: You don't need to upgrade, although you might want to. But the Series 3 is really the best value pick: at a reduced price, it's still really good.

Otherwise, the basic Apple Watch proposition, for the most part, is pretty similar to what it was last year.

We'll be updating this review-in-progress with a lot more tests to come as we wear it across both coasts.