The Central America-4 Border Control Agreement is a way for the countries of El Salvador, Guatemala, Honduras, and Nicaragua to work together to make it harder for people to just move from one country to the next without permission. All four of these countries have agreed that people who want to go from one country to the other need to get a special permission or visa from the government, like a passport. This helps all of the countries keep track of who is coming and going between them, and it also means that the countries are better prepared to catch people who are trying to do bad things.