jBPM4 is a complete rewrite, the closest die jBPM3's "task-node" is probably the "task" tag in jBPM4
http://docs.jboss.com/jbpm/v4/userguide/html_single/#task
to create multiple tasks in one node you might be better off by using a custom node
http://docs.jboss.com/jbpm/v4/userguide/html_single/#custom
override the execute() method and do what the TaskActivity is doing:
DbSession dbSession = EnvironmentImpl.getFromCurrent(DbSession.class);
TaskImpl task = dbSession.createTask();
task.setTaskDefinition(taskDefinition);
task.setExecution(execution);
task.setProcessInstance(execution.getProcessInstance());
task.setSignalling(true);