Police confiscated the explosives during a routine check of passengers with luggage at Frankfurt (Oder) train station in Brandenburg on Saturday afternoon.
The schoolboy had stashed over 700 bangers in his bag, which he had bought in a supermarket in the adjoining Polish city of Slubice - just over the border where the German restrictions on buying fireworks do not apply.
The law states that members of the public can only buy fireworks for three days each year between 29th and 31st December. This year, one of those days is a Sunday so sales will begin on the 28th.
"Quite often we find young people, teenagers or even adults carrying fireworks over the border at this time of year, this close to New Years Eve," Törsten Peters, spokesman from the federal police told The Local.
He added that 4.3 kilograms of explosives was quite a large amount for a 16-year-old foot passenger to be taking across the border, although people often attempted to smuggle larger supplies of fireworks hidden in vehicles.
Possession of bangers and fireworks - traditionally set off by revellers on New Years Eve - is illegal in Germany unless they carry the seal of approval from Federal Institute for Materials Research and Testing, said Peters.
Polish-bought explosives are often not checked and can be unsafe, resulting in countless accidents every year.
"The explosive devices sold in Poland often come from China, for example, and you never know how much explosive has been put in them," said Peters.