After 2004 polish exports increased when

After 2004, Polish exports increased after the country entered the European Union.

Poland, as multiple other countries of Central and Eastern Europe, has communist past. When the communism fell, Poland struggled for some time, as the country needed to change a lot in its functioning, especially the economy and the development of relations with other countries. Despite the country doing everything right, it was still not able to reach its potential because of the limitations.
This all changed when the country entered the European Union in 2004. The borders opened up, so Poland had access without any limitations with the other countries of the EU, and through it with lot of markets across the world because of the EU's trading agreements. Poland used the opportunity, so its exports increased significantly, and since there was larger market and demand, the Polish companies were producing more as well, skyrocketing the economy.
Countries that experienced similar development as Poland are:
Hungary
Lithuania
Latvia
Estonia
Czechia

Answer accretion;