"Since February 24, a total of 4,506,000 people have crossed into Poland from Ukraine," it said in a tweet.
On Saturday, around 25,700 refugees from Ukraine arrived in the country, compared with 23,600 a day earlier, according to the Polish Border Guard.
Meanwhile, more than 2.56 million people have left Poland for Ukraine since February 24, the agency also reported.
On Saturday, around 29,800 people crossed from Poland into Ukraine, the Polish Border Guard said.
A deputy interior minister said last month that Poland was home to some 1.5 million Ukrainians after many refugees from Russia's invasion of Ukraine returned to their country in recent 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.