What is the difference between the Iwatch 3 and 4?

Related questions

1. Can you change bands on Apple Watch?
2. Is it bad to sleep with your Apple watch on?
3. Does Apple watch bands work with Series 4?
4. Is the Apple Watch heart monitor accurate?
5. Can you change Apple Watch 4 bands?
6. Is a 40mm watch too big?
7. Can I use 42mm band on 44mm?
8. Why are watches worn on the left wrist?
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