See, airflow. Modification, by whatever means necessary, can make all the difference in the world. Having negative pressure airflow is fine and great if there are only exhaust fans, but without a good supply of airflow being brought in, which generally requires at LEAST one front intake, the exhaust fans will struggle to bring air in by way of pressure differential only. Of course, much depends on what the system is being used for and the degree to which the case is "sealed" too.
On a mostly stock system that will not be
1. Using a high TDP processor
2. Overclocking of anything
3. Running a high end graphics card
the need for cooling will be much less necessary and even a single rear fan and single front fan is likely to be sufficient. If it's a low enough TDP system, even just a rear exhaust is usually good enough so long as there is plenty of entry points for air to get in at the front/bottom of the case and preferably with at least some kind of venting or grillwork up top to allow passive dispersion of heat by way of radiant exchange.