According to chief organized crime prosecutor Daniel Horodniceanu, it is the largest seizure of heroin since the 1990's.
Prosecutors say an international heroin trafficking ring tried to transport the heroin, with an estimated street value of € 40 million (US$ 43 million), through Ukraine to Romania on its way to Western Europe.
It is thought the heroin originated in Afghanistan, with the likely final destination of the Netherlands.
On Wednesday, police said that the previous day they had stopped a truck driven by a 60-year-old man at the north-western Satu Mare border crossing.
They found packages of heroin concealed in the fuel tank.
The same day, another truck was stopped in the northern county of Bistrita, driven by a 61-year-old man. It was carrying packets of heroin that, like in the first truck, had been soaked with chilli pepper to confuse drug-detection dogs.
Both trucks are thought by police to have been sent by the same organized crime gang, which was already known to Romanian authorities.
The chief of the anti-organized crime unit, Jean Cucu, said they had monitored the group since 2010 as part of police operation "Green Arrow".
Five locations in Ramnicu Valcea were raided, resulting in the seizure of € 136,000 (US$ 149,000), two luxury cars, several mobile phones, computers and laptops.
Four detained were named as Hakan Yigit, Saman George-Solomon, Binga Liviu-Constatin, and Sincar Huseyin.
According to the US State Department's 2015 International Narcotics Control Strategy Report, drug traffickers often use Romania as an entry point into the European Union since it borders three non-EU countries.
