If you are aiming to lose weight, you should follow not only a good exercise routine but a low fat diet. Healthy food and some bike rides may be a good combination to lose weight, but you must be confident and persistent to see the results in some weeks.
On the other hand, you should not count miles (as 4 miles seems to be a short distance to go in a bicycle) you should count time and bike effort (hills are more difficult and represent more effort).