Here's how the new $400 Apple Watch Series 4 compares to last year's model, the Apple Watch Series 3

The Apple Watch Series 4 and Apple Watch Series 3 bear a lot of similarities. Both watches come with the option for GPS only, or GPS plus cellular connectivity.

There's a price difference if you opt to add a cellular connection — plus, the Apple Watch Series 4 is more expensive than the now-discounted Apple Watch Series 3.

Here's the price breakdown:

• Apple Watch Series 3 with GPS — $279
• Apple Watch Series 4 with GPS — $399
• Apple Watch Series 3 with GPS and cellular — $379
• Apple Watch Series 4 with GPS and cellular — $499

On the design front, there is a change from last year to this year in regards to the cellular version. The Apple Watch Series 3 with cellular had a red digital crown, while the Apple Watch Series 4 with cellular just has a thin red stripe on the crown. Apple also added haptic feedback to the digital crown on the Series 4.

Both watches get 18 hours of battery life, and are water resistant up to 50 meters.

There are a few key design differences between the two watches. The Apple Watch Series 4 has a bigger case size than the Apple Watch Series 3, and it also has a bigger screen.

The Apple Watch Series 4 has a slightly larger case size than the Apple Watch Series 3.

Both watches come in two sizes: the Apple Watch Series 4 in 40 mm and 44 mm, and the Apple Watch Series 3 in 38 mm and 42 mm.

The upside of the larger case, however, is a larger screen:

• Apple Watch Series 3, 38 mm — 563 sq mm display area
• Apple Watch Series 3, 42 mm — 740 sq mm display area
• Apple Watch Series 4, 40 mm — 759 sq mm display area
• Apple Watch Series 4, 44 mm — 977 sq mm display area

The Apple Watch Series 4 is thinner than the Apple Watch Series 3.

The Apple Watch Series 4 is 10.7 mm, while the Apple Watch Series 3 is 11.4 mm.

That being said, Apple's claim that the Apple Watch Series 4 is the thinnest ever isn't quite true — the Apple Watch Series 4 is still thicker than previous models of the Apple Watch. Both the first generation Apple Watch and the Apple Watch Series 1 came in at 10.5 mm.

The Apple Watch Series 4 comes in more colors.

Apple seems to have eliminated the gold Apple Watch Series 3, which means the Apple Watch Series 4 comes in more color options.

The aluminum models of the new watch come in silver, gold, and space gray, while the stainless steel models come in silver, gold, and space black.

The Apple Watch Series 3 only comes in silver and space gray.

Apple upgraded the internals on the Apple Watch Series 4. The new watch has a new heart sensor and can record electrocardiograms.

Apple added a new electrical heart sensor to the Apple Watch Series 4. The new sensor, along with an accompanying app, will be capable of recording an electrocardiogram, or ECG. Apple says that feature is coming later this year.

The Apple Watch Series 4 also has new tools to measure your heart rate, including a new low-heart-rate notification and atrial-fibrillation detection.

Apple improved the accelerometer and the gyroscope on the Apple Watch Series 4, and it now has a new fall detection feature.

Apple says the new Apple Watch can detect if you've fallen down and are unable to get up.

The Apple Watch Series 4 has a faster chip than the last model.

The Apple Watch has a new S4 chip, and Apple says the watch's 64-bit processor makes the watch two times faster than the Apple Watch Series 3.

But there's one major downside to the Apple Watch Series 4: it's significantly more expensive than the Apple Watch Series 3.

The Apple Watch Series 4 with GPS starts at $399, while the version with GPS and cellular costs a whopping $499.

The Apple Watch Series 3, on the other hand, is a relative deal. The GPS version costs $279, while the GPS and cellular option starts at $379.