Are your BA and Jet Airways flights on the same ticket? If so, you should get your boarding pass in LHR and your luggage should be checked through to your final destination.
Unfortunately DEL airport is quite far away from the city center, and at times DEL can be exceptionally busy, even in the middle of the night. With only 4 hours I wouldn't advise you to leave the airport. As sometimes it can take up to an hour just to get into the airport, because there are security checks before entering.