The agreement to expand cooperation in "strategic areas of the country" was made during a visit by US Defense Secretary Lloyd Austin.
It comes as the countries seek to repair ties that were fractured in recent years. Previous Philippine president Rodrigo Duterte favoured China over his country's former colonial master, but the new administration of Ferdinand Marcos has been keen to reverse that.
Beijing's growing assertiveness on Taiwan and its building of bases in the disputed South China Sea have given fresh impetus to Washington and Manila to strengthen their partnership.
Given its proximity to Taiwan and its surrounding waters, cooperation from the Philippines would be key in the event of a conflict with China, which a four-star US Air Force general has warned could happen as early as 2025.
The four new locations bring the total number of sites accessible to US forces to nine, Austin told reporters on Thursday.
Talks were ongoing for a potential 10th site, a senior Philippine official told AFP.
The announcement came as the United States reopened its embassy in the Solomon Islands after a 30-year hiatus as it competes with China for influence in the South Pacific.
The United States and the Philippines have a decades-old security alliance that includes a mutual defence treaty and the 2014 Enhanced Defense Cooperation Agreement, which allows US troops to rotate through five Philippine bases, including those near disputed waters.
It also allows the US military to store defence equipment and supplies on those bases.
The EDCA stalled under Duterte, but Marcos has sought to accelerate its implementation.
Philippine defence secretary Carlito Galvez told reporters the location of the new sites would be made public after local communities and officials had been consulted.
But it has been widely reported that most of the locations are on the main island of Luzon -- the closest Philippine landmass to Taiwan -- where the United States already has access to two bases.
The fourth will reportedly be on the western island of Palawan, facing the Spratly Islands in the hotly contested South China Sea, taking the number of sites there to two.
- 'Illegitimate claims' -
Austin said the allies were committed to "strengthening our mutual capacities to resist armed attack", as he accused China of making "illegitimate claims in the West Philippine Sea".
Manila refers to waters immediately west of the country as the West Philippine Sea.
Beijing responded Thursday, saying Washington was exacerbating "regional tensions" by continuously strengthening its military deployment.
The United States is also seeking to bolster alliances with other nations to counter China's rapid military advances, including its AUKUS partnership with Australia and Britain.
Australia has agreed to step up the pace of military interactions with Washington, while Japan is planning to enter joint exercises with both countries.
While Marcos has sought to strike a balance between China and the United States, he has insisted he will not let Beijing trample on Manila's maritime rights.
About 500 US military personnel are currently in the Philippines, with others rotating through the country for joint exercises as required.
- Protest against EDCA -
The US military presence has long been a sensitive issue in the Philippines. Around 100 protesters rallied outside the country's military headquarters on Thursday calling for the EDCA to be scrapped.
The United States previously had two major bases in its former colony, but in 1991 the Senate voted to terminate the lease agreement after growing nationalist sentiment.
Beijing claims sovereignty over almost the entire South China Sea and has ignored a ruling at The Hague that its claims have no legal basis.
The Philippines, Vietnam, Malaysia and Brunei also have overlapping claims to parts of the sea.
China also claims self-ruled, democratic Taiwan as part of its territory, to be reclaimed one day, by force if necessary.
"Looking at the location of the proposed sites, it seems pretty clear that these sites are in relation to a Taiwan contingency," said Greg Wyatt of PSA Philippines Consultancy.
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