"Since February 24, a total of 4,265,000 people have crossed into Poland from Ukraine," it said in a tweet.
On Wednesday, around 26,300 refugees from Ukraine arrived in the country, compared with 23,800 a day earlier, according to the Polish Border Guard.
Meanwhile, almost 2.29 million people have left Poland for Ukraine since February 24, the agency also reported.
On Wednesday, around 23,500 people crossed from Poland into Ukraine, the Polish Border Guard said.
A deputy interior minister said last week that Poland was home to some 1.5 million Ukrainians after many refugees from Russia's invasion of Ukraine returned to their country in the last few weeks.
Poland in March enacted a measure to offer wide-ranging support to Ukrainians escaping the Russian invasion of their country, which began on February 24.
The measure grants them residence rights and ensures access to education, healthcare and social benefits.
Nine in 10 Poles are in favour of accepting refugees from war-torn Ukraine, according to a survey.