The short answer
Hajin City is fictional. It is an invented South Korean city created as the backdrop for Modern Warfare 4's DMZ mode, and the wider Hajin Exclusion Zone around it is built from the same invented geography. There is no real Hajin City district in South Korea to visit or look up.
The setting is deliberately Korean: Korean place names, Korean signage and a city layout that reads as a South Korean coastal urban area. That is fiction with a real cultural reference, not a real location reproduced in a game.
Why people think it is a real place
Two things cause the confusion. The first is the name style: Hajin reads like a real Korean place name, and several of the major locations in the zone, such as the air base and the river heights, read like ordinary municipal geography.
The second is that search results for 'hajin city korea' mix real South Korean locations with DMZ coverage. If you are trying to work out whether the map you are playing is a real city, the answer is no: it is a purpose-built zone.
What is actually in the zone
The official tour names 13 major points of interest and says the wider Exclusion Zone includes more than 60 named locations. They range from an air base and a political prison to a shopping mall, a stadium and a nuclear reactor.
That mix is the point: the zone is designed so a deployment can start near a dense urban block and end at a rural edge, which is why the same run can feel like a street fight and an open-country extraction in one deployment.
Is it the same map as the old DMZ Beta?
No. The older DMZ Beta used different maps, keys and instructions, and MW4 DMZ is described as a rebuilt extraction experience with its own Forward Operating Base and persistent progression. Guides written for the beta will give you the wrong keys and the wrong locations for Hajin.
If a guide refers to an older DMZ map by name, treat it as outdated for MW4. The MW4Field index only lists locations that appear in the official Hajin material.