I will cut to the chase. Unfortunately, the Mi Band 3 charger isn’t compatible with the Mi Band 4. And it was to be expected. The Mi Band 4 is larger than the Mi Band 3 and hence it doesn’t fit inside the charging cradle which was specifically made for the fitness tracker.
However, you shouldn’t be too worried in case you have lost your Mi Band 4 charger. You can get a genuine charging cable for as low as $5 at Aliexpress. And in case you have got a Mi Band 3 charger laying around and hoping to get it charge the Mi Band 4 somehow, you are out of luck. Mi Band 3 comes with charging pins on the side whereas the Mi Band 4 charging pin is located at the back. Thanks to this change, you really can’t get the Mi Band 3 charger to work with the Mi band 4.
Where can you buy a genuine Mi Band 4 charger?
If you have lost your Mi Band 4 charger and are looking for a new charging cradle for the Mi Band 4, the two best places for you to get a replacement charger is Aliexpress and Amazon.com. You will see that Aliexpress usually has products for lower price but you will have to bare in mind that it can take 20-30 days for the seller to ship the charging cradle for your Band 4.
If you live in the US, Amazon is going to be a better bet since you will have easier time getting a replacement in case you receive a defective unit and also the shipping time is going to be much faster.
If you are living anywhere else where you can buy Mi Band 4 charging cradle officially, it’s best for you to buy the Mi Band 4 charger from Aliexpress. Just note that shipping can take around 20 days.