Thank you all for your responses. In summary, from what I have taken from the answers here, I take this to be the explanation:
The use of 4 filters increases the Filter face area(the box length and width of the filters- no stretching out the pleats) of the filter medium by 4, so the Filter face...
Typical box fan+air filter setups use axial consumer-grade fans designed for providing maximum air flow at low power consumption. These fans do not provide sufficient air flow at the high static pressure requirements typical of air filters. The air filters DIYers use are high Merv(e.g Merv 13)...