Pouring at the pressure I want to pour at...

So I've kegged my wheat, and pressurized it to about 3 volumes. I did so around 18psi. My question is, I want to pour at around 4psi, but since my keg is at 18psi, it won't let me pour at 4psi because the keg is already pressurized to 18psi inside. So, the only way I can think to fix this is to release some of the pressure in my keg using my pull pin. Is that correct? Won't it also be releasing the carbonation in my beer that I've been waiting for?

Lowering psi will cause pouring issues and carbonation issues. The beer will lose carb equalizing with the psi ffrom the tank.

Why do you want to serve at 4psi? If you need a slower pour, get longer lines.

carbonation level is a function of temperature and pressure. if you want it at 3 volumes and 18psi, you should have carbonated it at 40 degrees.

if you want to keep the same number of volumes of carbonation but drop the pressure down to 4psi, you will need to chill the beer to well below freezing, which isnt posssible.

even at 30 degrees F, you need to keep it at 12psi to keep 3 volumes of CO2. there is no way around this.

if you want to pour temporarily at 4psi, you can vent the CO2, pour, and then when you are done put 12psi back on the beer to keep the proper carb amount. otherwise, get longer liquid lines so you can increase the pressure.

So, what are the line lengths would you recommend?

Start with 12 feet and trim as required to get the pour you desire. My best guess is that you will need just over 10 feet to get a balanced pour at 18 psi.

This page may help:
http://www.iancrockett.com/brewing/info/kegbalance.shtml

Cheers!

BTW, be sure to use 3/16" line.