I serve my heavily carbonated Hefe's through my 12' line with no problems. 4.0 volumes is too high for a hefe in my opinion. More like 2.7 to 3 volumes max. 18 psi will get you 3 volumes at 40˚F and will serve fine through 12' of line, considering the rule of thumb of 2 psi drop in pressure for every 1' of 3/16" ID thick walled beverage tubing. Even if you have a line length that mathematically would cancel out flow, based on the 2 psi/ft rule of thumb, you would still have flow, albeit very slow.
Root beer, on the other hand, is another story! The high sugar content and high volume of CO2 cause it to foam severely during a pour with a standard length of beer line. The rule of thumb of 2psi/ft does not work well with soda, if carbonated to the levels we are accustomed to with store bought soda. I keep my root beer set to 30psi and serve it through 35' of beverage tubing and get a nice, slow, perfect pour with minimal foaming.