I agree that the snacking thing is important to tackle first. My son, 4 mths old, was also a snacker and eating every 2 hours for about 10 mins... so slowly over the last week, I have been encouraging him to take more at each feeding by making sure to burp him often between breasts and being sure to offer the breast over and over until I am sure he is really not interested any more. Gradually, this helped him extend the feedings and he is slowly working his way to four hours... There may also be times in the day where he will only go 3 hours but later in the day go 3.5 or 4 hours... so that is a possibility too.
Hope that helps.