CUPERTINO, Calif.—Apple executives took to the stage at the Steve Jobs Theater on Wednesday to announce the new Apple Watch Series 4 alongside other new devices.

The follow-up to last year's Series 3 Watch eschews a complete overhaul for subtle changes that increase its screen size while adding other improvements to the watch's hardware.

The device will be available in silver, gold, and space gray colorways and ships to consumers on September 21, with pre-orders starting on September 14. It will start at $399 for a GPS option and $499 for a model with GPS and cellular connectivity. Apple says all prior Apple Watch bands will continue to work with the Series 4.

The Apple Watch Series 3, meanwhile, will stick around and start at a lower $279 price point.

The Apple Watch Series 4 comes in 40mm and 44mm case sizes, and Apple says the displays on both are more than 30 percent bigger than previous models. A new watch face can hold up to eight customizable complications, the Apple Watch version of apps, at once. Apple says the new space can be used to connect with contacts with a tap, show different time zones at once, or see more detailed data related to sports scores, health, fitness, maps, and the like. Apple says it has made a new watch face dedicated to the existing Breathe app, too.

Apple says the device is thinner than the Series 3, however, giving it a smaller overall footprint than before. The company says the device will retain the same 18-hour battery life as the previous Watch, though we'll have to put the wearable through its paces to see exactly how it stacks up.

The wearable's speakers are said to be 50 percent louder as well, which Apple says will make the device better for taking phone calls and talking with the Siri digital assistant. Apple says it has moved the Series 4's microphone to the opposite side of the speaker to reduce echo. The device also has a re-engineered digital crown button that includes haptic feedback. The back of the device, meanwhile, is said to be made of black ceramic and sapphire crystal. All of this remains swimproof, just like the Series 3.

The whole thing runs on Apple's new S4 SiP, which includes a dual-core 64-bit processor and a new GPU. Apple says the device will have up to 2x the performance of the previous model. The device also includes a faster accelerometer and gyroscope with twice the dynamic range.

Apple also touted a few features related to the Apple Watch Series 4's ability to monitor heart health. The company says the device will send you a notification when it detects your heart rate to be too low or if it detects an irregular heart rhythm.

Most notably, the Series 4 includes an electrical heart sensor that allows users to take an electrocardiogram, or ECG. Apple says the feature can be used by placing a finger on the digital crown and that the whole process takes around 30 seconds. It's worth noting that medical ECG sensors use multiple electrodes on different spots on the body, so we wouldn't expect the Series 4 to be as accurate on its own. But Apple says the Series 4 does have clearance from the US Food and Drug Administration (FDA), which is a first for a mainstream smartwatch. In any event, tracking electrical signals from the heart should result in more accurate readings than measuring a user's pulse from the wrist, and the feature will only further Apple's push into more medical territory with its flagship wearable.

Beyond that, the device includes a fall-detection feature, which Apple positions as particularly helpful for elderly users. The company says the Series 4 will deliver an alert after detecting a fall, after which a user can initiate an emergency call. If the Watch detects no movement after one minute, Apple says the device will then make that emergency call automatically and send your location to a designated emergency contact.

The Apple Watch Series 4 will run on watchOS 5, the latest version of Apple's wearable operating system. The update includes features like automatic workout detection, Walkie Talkie messaging over Wi-Fi or LTE, and compatibility for Siri shortcuts.