Keep in mind that if you do quench and temper you cannot use the procedure for stress relief production regimes. You have exceeded transformation temperature. A quench and temper is not a stress relief, although it will do that.
Matt,
Your hold time of one hour per inch sounds to be a standard rule of thumb. You might be having an issue with your fall rate. Are you able to monitor and Control how fast you are cooling down? Thermocouples in use, or are you just peeling back the insulation and using a gun or temp stix? An eternity of hold at temp is of little benefit if the zone gets a quench effect. Stuff the ends so there is no internal draft. Air blast can be just as detrimental as dunking in the bucket (well, not quite). 4140 tends to be some finiky stuff.