port volume/tuning

You did the conversion correctly...for port volume, that is, but that isn't what you are looking for.  The slot port opening you described is almost twice the square inches as the round port's opening.  A 4" round opening is 12.56 sq in.  The slot's is 24 inches.  So, what happens when you increase the opening size, in regard to port length?

The length of the port grows longer when you increase opening size if your intent is to maintain the same tuning freq.  In this case with your calculations, your new slot port's tuning is much higher than the round port's would have been. 

What you should do instead is find the square inches of a round opening - 4" is 12.56 sq in. as I said above - and use that 12.56 area to arrive with slot (rectangular) dimensions.  Determine a long side, say 8" for example, and divide that into 12.56 to find the short side.  12.56 / 8 = 1.57   You can't use 12" for the long side because then the short side would be too narrow.

Find the square inches of a circle by using pi r squared ( this thread has a full explanation).

So for the same tuning frequency your slot should be two dimensions that equals 12.56 square inches (such as 8" X 1.57") and should have exactly the same length as the given round port's (20.5").  Increasing port opening size will indeed make for a quieter port but you will have to give up much more volume for the increase in length.

The power you expect to provide the subwoofer will determine how large the port opening size should be.  A 4" round or equivalent slot is usually suitable for one sub in a vehicle situation, but multiple woofers or high powered situations call for an increase in the port opening to avoid port noise.  Often a compromise has to be made in port opening size due to the long length of the port for desired tuning.  In this case, at 20.5" L with a 4" diameter opening, you've come to a point where it would likely be unreasonable to make the port longer.  So you stick with the 12 1/2 square inch opening if it doesn't make unusually loud port noise.  A modeling program (or the math itself) is really needed to find this out.  If nothing else, you go with the manufacturer's suggested size. 

Besides WinISD, where you can find a tutorial here on this site, there are other calculators available online.  Here are some that may help:

• http://www.subwoofer-builder.com/freesoft.htm
• http://www.madisound.com/services/hifispeakerdesign.php
• http://www./help2/Articles.cfm?id=31
• http://www.ajdesigner.com/phpsubwoofervented/efficiency_bandwidth_product_equation.php
• http://www.doctorproaudio.com/doctor/calculadores_en.htm
• http://www.linearteam.dk/default.aspx?pageid=unitconverter

It would be longer than 21".  I don't know exactly what.