The group, like everyone, has specific shows (and days) off. (I saw what they were once, but can't recall).
I assume you were just unlucky enough to go to some shows at those times.
They used to have a female quartet that performed when the guys were not there, but I haven't seen the gals' group for quite some time.
So now, when "4 for a Dollar" are not performing, there is no preshow.