A fourth order enclosure (vented box) is a sealed enclosure with the additon of a port tuned to a specific frequency. The port extends the low frequency response of the basic sealed enclosure usually providing a lower cut-off frequency. The enclosure offers a good transient response (second to that of a sealed enclosure), good power handling within it's bandpass region, and excellent efficiency making it a popular choice for many competitors and everday users, but below the tuned frequency power handling is poor and damage to the woofer is likely to occur if over driven. Lowering the port frequency below that of the manufacturer's recommended frequency will only worsen matters and cause the sound to become "muddy"....bleah!
that comes from a well respected source //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif