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.