While these black squares may look outwardly similar, X-rays from our experts at Creative Electron reveal radically different insides.

Already our subject (left) presents with some differences from the Series 3 (right)—the back sports an all-new optical sensor in the middle, surrounded by a new electrode.

We're looking forward to a closer look at that ECG hardware.

Other immediate differences from the "old" 42 mm version: the corners are noticeably rounder for more of that squircle look, and the side button now sits flush with the case.

Seen from the side, the Series 4 is noticeably slimmer than the Series 3 (by 0.7 mm)—but it's actually thicker than the original.

From the look of things, a good deal of that slimming is thanks to the new display.