China's national meteorological centre cautioned on Sunday that new downpours were expected in parts of Liaoning for another 24 hours at least.
A couple in their 70s and a mother and son died in Kuandian county, around 100 kilometres northeast of Dandong, when flash floods swept away their homes, China's official Xinhua news agency said, citing a local flood-control official.
A 60-year-old man was also missing in Kuandian after his house collapsed in a rain-triggered landslide.
Nearly 3,900 people have been killed or left missing this year in flood-related incidents across China, official figures show.
In the northwestern province of Gansu, a mudslide on August 7 crashed into homes in the remote town of Zhouqu, leaving at least 1,434 people dead and another 331 missing.
In the southwestern province of Yunnan, rescuers are still searching for 69 people who went missing in rain-triggered mudslides in a remote, mountainous area.
Twenty-three people have been confirmed dead, Xinhua said.
North Korea evacuations
In neighbouring North Korea, more than 5,000 people have been moved to safety after parts of Sinuiju city and rural communities near the border were "completely inundated", the official Korean Central News Agency said.
North Korea has been hit by widespread flooding this summer, which has washed away homes, roads, railways and farmland, causing an unspecified number of deaths, according to state media reports from Pyongyang, the capital.
After decades of deforestation, North Korea is particularly vulnerable to flooding. In 2007, it reported at least 600 people dead or missing from devastating floods.