No. Its not that simple, the birds have be in the phase of the reproductive cycle where the presence of a chick illicits a protective response. For some birds, this is most of the time, for others not so much. Trey and Princess are experienced adults and they turned down my offer of foster chicks last year, whereas the pair on William Osler in Ontario took over pre-fledging chicks from another TO site this year. (Hope I'm remembering the locations correctly). And there is no reason to take a fourth chick out of a nest, four is a standard nest number - if all the other chicks are fine, then there is no reason to believe that the fourth one is being neglected (peregrines don't practice infanticide).
I'll go see if I can't spot the chick and report back later ...