"These cities are in densely populated eastern locations that are close to areas with developed industrialization and a robust commercial economy, and also have close ties to central cities," Ren said.

According to the Seventh National Population Census information released in 2021, the permanent population of Kunshan was 2.09 million, Yiwu 1.86 million, Cixi 1.83 million and Jinjiang 2.06 million.

The four county-level cities are all close to megacities or located in metropolitan circles, which demonstrates an important path for future urban evolution, said Zhong Huiyong, an associate professor at Shanghai University of International Business and Economics.

"Rising population growth means that the big cities become more agglomerated. That can provide more employment opportunities in the cities, and average wages may also increase," Zhong said.

The four cities were all listed among China's top 10 counties and county-level cities in GDP in 2021.

Kunshan, with a gross domestic product of 474.8 billion yuan ($66.55 billion) in 2021, has ranked first among the top 100 counties and county-level cities in China for 17 consecutive years. Cixi became the top county in Zhejiang with its GDP reaching 237.9 billion yuan in 2021. Yiwu's GDP in 2021 hit 173.02 billion yuan, with a growth rate of 11.6 percent year-on-year, which is the fastest among the top 10 counties and county-level cities. Jinjiang's GDP in 2021 reached 298.64 billion yuan, with a growth rate of 10.5 percent year-on-year.

The future development prospects of the four cities have also attracted attention, given that they are positive examples of urbanization for other county-level or small cities across the country.

"Being regarded as a big city impacts the urban space. Urban planning is largely based on urban economic development, especially changes in urban population," Ren of Fudan said.

He added that the growth of the urban population and its change in scale exerts pressure on urban management and services, and also creates new demands on urban planning and public services, such as public transportation, education and public facilities.

"Each city needs to find its developmental path with local characteristics," Zhong said.

"It is necessary to know what the comparative advantage of the local industry is. After finding the comparative advantage, each city needs to consider development according to local conditions."

Also, for other county-level cities in coastal areas, it is necessary to establish more interconnections with surrounding megacities, Zhong added.

Fang Xiaoying contributed to this story.