Polish Deputy Prime Minister and Defence Minister Władysław Kosiniak-Kamysz signed the agreement in Warsaw, alongside US Undersecretary of State for Arms Control and International Security Brent T. Christensen.
"We have reaffirmed the Polish-American alliance and are setting an example for Europe in terms of defence spending," Kosiniak-Kamysz said.
"We are ready to spend this money to transform the Polish army and strengthen Poland’s security," he added.
The loan guarantee is being provided under the US Foreign Military Financing (FMF) programme, which offers loans and guarantees to allied countries for the purchase of American-made defence equipment, Polish state news agency PAP reported.
Kosiniak-Kamysz said the funds would help Poland build "a strong army capable of tackling challenges."
According to the US embassy in Warsaw, the new agreement brings total FMF support to Poland to more than USD 15 billion over the past two years.
"This marks another milestone in the strategic relationship between the United States and Poland," the embassy said in a post on X.
"The USD 4 billion loan will further modernise the Polish armed forces and strengthen Poland’s role as a pillar of NATO's eastern flank," it added.
The embassy said the agreement underscores the US commitment to its partnership with Poland, and reflects Poland’s growing role in NATO and regional defence.
In recent years, Poland has received US loans and guarantees to help finance the purchase of equipment such as Patriot air defence systems, HIMARS rocket launchers, Abrams tanks and F-35 fighter jets, public broadcaster Polish Radio’s IAR news agency reported.