I would like to develop a wearable with a BLE5 module. I wonder if this module would work with any BLE4 enabled mobile phone? I am aware with that it may not use the advantages of BLE5, but I don't want to develop something that will not be used for a while. I read a similar question for BLE4.1/BLE4.0 compatibility, and answer was yes. However I am not sure about major version number changes are also backward compatible. Do you have any experience with that?
Yes, as long as you are not using the one of the new Bluetooth 5 features (2xSpeed, 4xRange, or LE Advertising Extensions), then your wearable will work with Bluetooth 4.x devices.
P.S. You will probably be using GATT for exchanging data; i.e. your wearable will be the GATT server hosting a GATT table for remote phones/tablets/PCs (GATT clients) to read the data from. Familiarise yourself with these terms before starting your project :-)
just proven now myself, Mi Band 4 with Bt5BLE cannot receive pairing data from samsung A7, but A7 can read pairing from Mi Band 4. So the Pair is broken. "compatibility" is an utopia, or a Sales Gimmick. Early Adopters will simply become the victim of "such compatibilities". End of story. Lets wait until all phones and BT dongles move to BT5BLE. Put that Mi Band 4 in the fridge for the next 2 years, or buy a newest BLE5 phone.