How can I charge my beard trimmer's Li Ion battery from USB?
Li-ion batteries are usually charged by specific charger controller IC, which tracks charging current, voltage of battery, temperature, etc.

It all depends where the charging controller is located - if it's in the device, you can get away by feeding it with other voltage than 4.2, if it's in a charger, well, you'll have to make yourself a proper li-ion charger. 

no way to tell without opening in any case...