Is BLE 5 backwards compatible to BLE 4.x ? Can a phone with bluetooth 4.x talk with a device with BLE5
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 :-)