According to BBC News the 15-minute peak for Derren Brown was 4.6m, suggesting that the audience was fairly stable throughout the hour. I assume the 5m figure is the 5-minute peak.
I think it was a very good move by Channel 4 and Brown not to say "how" he did it last night. Had he revealed it last night, chances are most would have forgotten about it and viewing figures next week would go down to about 2.5m.
By not revealing it, the suspense stays and therefore so does the publicity - and hopefully the ratings. If they can get 4m next week, that would be superb.
The concern BBC might be having is that this publicity stunt by Channel 4 may in turn dent Strictly Come Dancing. I suspect it will still win its slot, but it's actual average will be quite low - I'd say about 7 million only will tune in. Three reasons:
a) Friday night - will everyone know about Friday launch, or will some sections of fans forget or be out?
b) Corrie clash. Soap fans are use to turning from ITV1 to BBC1 to ITV1 on Friday's, so they will need to promote Strictly directly before EastEnders on Friday.
c) Tough 9pm slot. It depends a lot on how the Brown publicity machine works this week, really.
I suspect the breakdown will look something like:
20:30 to 21:00 - 5.9m
21:00 to 21:30 - 7.9m
21:30 to 22:00 - 8.5m
Average = 7.4 million
I really cannot see 9 million plus. That slot for shows like that (excluding BB/IAC/BGT) is untested.
Given BBC's figures the past few Friday's have not been good, I'm not sure if it will work as it could give The X Factor a head start.