Apple's iPhone XS, as good as it is, isn't all that different from its predecessor, the iPhone X -- even on the inside.

But the company's Apple Watch Series 4 is a major update compared to Series 3, and it's all the more apparent when you tear it down to bits.

We already knew that the Watch 4 is slimmer compared to Watch 3, with a bigger screen and new heart-rate sensor. In this detailed teardown, conducted by iFixit, we see just how different the two devices are; just look at the x-ray photo, below.

The circular thingy in the middle is the new optical heart-rate monitor/electrical heart sensor combo. Numerous other details are different; for example, the screen of the new Watch has slightly more rounded corners, and the taptic engine is a big longer but also thinner.

The teardown also reveals that the Apple Watch Series 4 has a bigger, 291.8mAh battery, a 4% capacity increase over Series 3, and that's despite the Watch being thinner by 0.7mm.

Most impressive, overall, is the sheer amount of tiny components Apple managed to elegantly squeeze into a wrist-worn device.

Amazingly, despite the number of components involved, Apple Watch Series 4 scores pretty well on iFixit's repairability scale: 6 out of 10. Both its screen and battery can be replaced (operative word here is "can" -- don't do it unless you know what you're doing), and band replacements are simple and backward-compatible.

Check out the full teardown here.