It may be that FIVE have booked advertising time on Channel 4. There is a difference between an advert and a trailer. Sky and Virgin advertise on other channels.
Big Brother is most likely made for FIVE by Endemol, the same company that made the programme for Channel 4, they own the rights to the show.
Channel 5, not Endemol, have paid for the commercials on other commercial channels. Endemol have no need to advertise since they already have a 2-year contract with five which will see them receive £200 million even if no-one watches the show.