Will the Mi Band 3 band fit the Mi Band 4?

Unfortunately, the Mi Band 3 replacement band won’t work with the Mi Band 4. Mi Band 4 has different dimension than the Mi Band 3.
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Does the Mi Band 4 has GPS built-in?

I will answer the answer you are here for right away. No, the Mi Band 4 doesn't have built-in GPS on-board. Just like the Mi Band 2 and the Mi Band...
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Mi Band 4 Chinese vs Global Variant: Differences?

If you have been shopping for the Mi Band 4, you should have noticed that there are two variants of the fitness tracker in offer: the Mi Band 4 Chinese variant...