The engines with the little ducts at the top are fitted with turbo compressors for cabin pressurization.
It can vary somewhat, I think you could choose from having 2, 3 or 4 on the aircraft.
One with 4
Photos: Boeing 707-437 Aircraft Pictures | Airliners.net
One with 3
Photos: Boeing 707-321B Aircraft Pictures | Airliners.net
One with 2
Photos: Boeing 707-123B Aircraft Pictures | Airliners.net
I understand that the three I picked out are all different series but I believe if you search further you will find variations with in the -100/-300/-400 subgroups.