That shouldn’t be the case if other conditions are the same. Beware that not only the number of sprite matters, but more importantly how large are those sprites. If you have 300 sprites but each one of them covers the whole screen you will not get a good performance.
Please attach some internal profiler log/screenshot or share the link of the test project. It would help to identify problem better.
We are making an action puzzle game with ex2D, with hundreds (normally over 400) of sprites on screen and tons of needs for updates. And we get a solid 55fps on iPhone4. So I don’t think there’s a particular performance problem with ex2D on iPhone4.