We also have to take into account that families have had to shell out a lot of $$$ to see Jurassic World, Superman AND Fantastic Four back to back in the span of like, a month and change. Movie-going has become hella expensive. A bag of Skittles is $15 in NYC. It could be that people are simply exhausted financially and are conserving costs for other family things like back-to-school needs, last summer vacations with the kids, etc.
It may have been a mistake for Marvel to place F4 last so tightly behind the other big hits. However, since the WOM is great and the film opened so highly, I’m inclined to believe that the GA may simply have other priorities at the moment this particular week and may come back to catch the film in theaters in the weeks to come. All is not lost.