How can I search for cheap flights during a window ~4 weeks as opposed to choosing specific days?

Every site I've seen so far requires actual days to be entered. I'd rather see the information that shows what would be cheapest over the period of around 4 weeks and tailor my trip to the best deal.How can I search for cheap flights during a window ~4 weeks as opposed to choosing specific days?
Some airlines allow you to look at dates before and after the date you choose. BA does. US airlines do as well if you are flying popular routes. Otherwise you will have to put all the dates in and see what the prices are.How can I search for cheap flights during a window ~4 weeks as opposed to choosing specific days?
Air India offer a cheap flight always. they serve only water in flight, the cabin itself smell of chilly powder and curry foods, the flight attendances release gas all the time,... but on the other hand at least you have a cheap flight! :)