What is the equivalent of register-defaults="false" in Spring 4?

Your Java Configuration as it is, is similar to the one with register-defaults="false" in XML Configuration. You don't need to do anything. But if you want to Register Defaults you would have to extend from WebMvcConfigurationSupport and add the last line specified in the below code snippet.

@Configuration
@EnableWebMvc
@ComponentScan
public class TestDataConfig extends WebMvcConfigurationSupport {

    @Override
    public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
        final MappingJackson2HttpMessageConverter converter = new MappingJackson2HttpMessageConverter();
        converters.add(converter);
        super.addDefaultHttpMessageConverters(); // Have to call this explicitly to register Default Message Converters.
    }
}