4" barrel in Kahr
I can't give you a diffinitive answer, but I might know how to find out. Take a look at the part numbers for a replacement barrel on the K9 and compare it to the part number for the replacement barrel on the MK9. If they are the same, then the K9 4" ported barrel will just drop in. You can find this by going back the Kahrshop.com and searching the parts side. It takes a few minutes and you'll have to write the numbers down so that you don't forget when you go to a different pistol, but this should answer the question. If they are NOT the same number, I wouldn't try it without a gunsmith being involved: and then your MK9 would probably always have the 4" barrel.
FWIW
jason