I really can't believe that gas pressures at the meter will fluctuate according to usage. The delivery pressure on a low pressure meter is 8" wc. I know that winter to summer the main gaslines will fluctuate from 30 to 60 psi. I have never seen a meter pressure change from summer to winter if the reg was properly setup. As far as saying that it is only .5" difference from 3.5 to 4" wc. Put a manometer on the valve and I can guarantee that when you adjust your regulator from 3.5 to 4" you will see and here a difference in the burners. As far as a unit being designed to run at 4" instead of 3.5" it will state what pressure is required on the unit. You may have to derate it for altitude but other than that always use manufacturers recommendations.