Here is what you need to do for the transfer to Jet Airways even if they are on the same ticket.
1. Do not use the transfers when you arrive on BA that is for Air India passengers.
2. Go down the escalator and go through immigration.
3. The first and business class are at the start and there is a visa on entry line at the very end. Better to check before you stand in line for too long.
4. After you are done with immigration collect your bags.
5. Exit the terminal through green channel assuming you don have any big ticket items.
6. Once you exit the green channel through the baggage doors "DO NOT EXIT" the main terminal doors.
7. On the left you have the plaza premium waiting lounge suggest you get some rest there.
8. Hour and half before your flight there is an elevator on either side of the arrivals terminal to go up to the check in area.
9. You will need to have the details that shows your name, date and flight number of the your Jet Airways flight either printed, email, or boarding pass.
10. Security guy will check your detail and send you up, and you will be checked again at the top before you are entered to the main departure / ticketing area.
11. Jet Airways is around B or C in the checkin area.
12. Drop your bags off get your baggage drop off receipt and head through security.
Delhi is a very simple airport to transit around. You can stay in the air conditioned zones the entire time. There is an insane amount of double quadruple triple checking of your documents. Other than that is pretty good. If you are in Business in Jet Airways the food is pretty decent and they have a lounge upstairs right after security.
If you have four hours do not leave the Airport the transportation and traffic can be a nightmare. It will take you one and a half hour to get into center of Delhi at that time.