I also thought Fantastic Four 2 would be in that same range. I was suprised at first that it didn't open up bigger, but then again I guess when the first movie in a franchise is generally not liked much, and the second isn't all that different from the first - expecting different, or better, results is asking for a miracle.
Along those same lines, I don't think Transformer's 2 can be as successful as 1 without clearly being a better movie. I really enjoyed 1, but the next movie cannot ride the wave of special effects the first one is doing. It will need much better character development, a more epic battle between Prime and Megatron, and a few new Transformers.