That does explain the scene of Sue crying over Johnny and the scene of Ben diving as The Thing, both scene in trailers/tv spots.
"Too bad YOUwill die." -Worst. Line. Ever.

The original ending according to Fantastic 4 movies wiki
Rather than Franklin dying, it was Johnny who ends up getting injured while fighting Doom, resulting in him getting crushed by a boulder controlled by Doom's telekinetic abilities.
An alternative ending was shown that Doom escapes from Planet Zero and ends up in his home country of Latveria where he is worshipped as a God in the original script. Johnny survives his injuries, and tells the team that this is only the beginning. Ben volunteers to dive bomb into the country, to apprehend Doom, but before long, Doombots start to appear. Ben then says his famous line before running towards the camera where it cuts to black and the credits start.
"Like my great grandpappy used to say, when life gives you booty, make bootyade!"-Big E of New Day