There are quite a few reasons why your 4 cyl car is getting worse mileage than the V6 car. First one would be how much time do you spend on your commute accelerating from a stop to high speeds? A 4 cylinder car will give worse gas mileage than a V6 if this is a common thing, since it has to work harder to make the same speed in the same distance.
Next up would be typical maintenance items, like your air filter, fuel filter, and even the brand and size of tires between the two cars. Different tires between the two cars can really change the fuel mileage results.
BC.