Regarding a single battery:
Professional overcharging equipment can also charge at a higher voltage. I also overloaded 30V.
However, the 30V for overload refers to the external voltage. The cell voltage is always less than 5V.
As for batteries in series, four batteries can be fully charged and connected in series, but you will never get 16.8, only about 16.5 at most. Since a qualified charger cannot charge 4.2V, it can only charge a few 4.1V.