C4 diagrams are essentially a standardised and formalised way of using other diagrams, as such the syntax is similar to other diagrams. Since there is only experimental support it may change, but here's an example anyway:
C4Dynamic
title Dynamic diagram for Internet Banking System - API Application
ContainerDb(c4, "Database", "Relational Database Schema", "Stores user registration information, hashed authentication credentials, access logs, etc.")
Container(c1, "Single-Page Application", "JavaScript and Angular", "Provides all of the Internet banking functionality to customers via their web browser.")
Container_Boundary(b, "API Application") {
Component(c3, "Security Component", "Spring Bean", "Provides functionality Related to signing in, changing passwords, etc.")
Component(c2, "Sign In Controller", "Spring MVC Rest Controller", "Allows users to sign in to the Internet Banking System.")
}
Rel(c1, c2, "Submits credentials to", "JSON/HTTPS")
Rel(c2, c3, "Calls isAuthenticated() on")
Rel(c3, c4, "select * from users where username = ?", "JDBC")
UpdateRelStyle(c1, c2, $textColor="red", $offsetY="-40")
UpdateRelStyle(c2, c3, $textColor="red", $offsetX="-40", $offsetY="60")
UpdateRelStyle(c3, c4, $textColor="red", $offsetY="-40", $offsetX="10")
C4Dynamic
title Dynamic diagram for Internet Banking System - API Application
ContainerDb(c4, "Database", "Relational Database Schema", "Stores user registration information, hashed authentication credentials, access logs, etc.")
Container(c1, "Single-Page Application", "JavaScript and Angular", "Provides all of the Internet banking functionality to customers via their web browser.")
Container_Boundary(b, "API Application") {
Component(c3, "Security Component", "Spring Bean", "Provides functionality Related to signing in, changing passwords, etc.")
Component(c2, "Sign In Controller", "Spring MVC Rest Controller", "Allows users to sign in to the Internet Banking System.")
}
Rel(c1, c2, "Submits credentials to", "JSON/HTTPS")
Rel(c2, c3, "Calls isAuthenticated() on")
Rel(c3, c4, "select * from users where username = ?", "JDBC")
UpdateRelStyle(c1, c2, $textColor="red", $offsetY="-40")
UpdateRelStyle(c2, c3, $textColor="red", $offsetX="-40", $offsetY="60")
UpdateRelStyle(c3, c4, $textColor="red", $offsetY="-40", $offsetX="10")