this happened to me on a first-time pairing of a S0 to my iphone6. 4h is just about the length of time the watch will run if some app is continuously running in the background. while it's a "microsoft" solution, apple's technical support had me unpair and repair the watch, but set up as a new watch, and then warned me to add back apps 1 at a time. the fact is that i don't really need any apps on the watch, so i just skipped that. after the repairing the watch has been fine - about 12h runtime in normal mode and around 24h runtime in theater mode.
i have a feeling the Kardia app might have screwed things up, as it just gave a spinner for a while and then told me that it was not approved by the FDA in the US.
anyway you probably have to do the same thing. major PITA and if you are using apps then an even bigger PITA.