I can only get 3 on the back of my longtail (Surly Big Dummy). Could attach a trailer or ask one to ride? You can bag and drag a bike pretty easily, even with someone sitting on the bike, although not so much if they’re in a proper children seat (the type with straps and footrest).
I used to have one child up front on a weeride but my youngest is nearly 4 and pretty big, she doesn’t fit up front. Probably limited to about the age of a big 2 year old.
The best thing is it rides a lot like a normal bike when no one is on it and it’s narrower than a box bike. But I do think you’ll struggle to get 4 on a longtail, the rear rack just isn’t long enough (plus the handling wouldn’t be great). I tried two adults and a child on the back once and it was a bit wobbly…!