Why would a dryer take 4 hours to dry after we cleaned out the vent?
After cleaning the dryer vent you should be seeing shorter drying times per load. A restricted dryer  vent limit's (reduces) the amount of warm, moist air being exhausted; slowing drying time. As already mentioned check again for lint blockage. If the thermostat is defective it will not fully heat-up. Put your hand on the exhaust duct; it should  be warm. Dryers employ a blower to circulate the warmed air. If the blower's drive pulley is loose it can't blow air. If the dryer uses electricity to dry clothes the heating element could be damaged.