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 :-)
BLE 5 is backwards compatible with the current versions.