two 4" ones would need to be like 15.5" long (after port displacement wich is .3 cubes). so i dont really think you can do that... unless you wanna get some plastic elbows.
now - the tuning would be about 33hz with these, but i dont think you would have port noise. you CAN go with a single 4" one... but id rather have two 3" port than one 4" port... and with the two 3" ports, youd have a lower tuning frequency too.
but again - it wouldnt be EXACT - because the thickness of the tube is unacountable to guess (some PVC is thicker than other) but its such a small displacment it dosnt matter.
and mind you - there is quite a hump in the output... **** near +4db at 53hz... so i dunno. but it will be way louder than sealed. i can guarantee that. the cool thing about tube ports. is you can take them out, cut them longer, or make them shorter and get a different tuning //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif