This topic is not about new features but changes, primarily in the object model, that will affect scripts that run perfectly well under CS3.
Bear in mind that as before, you can create a folder for previous version scripts and run from there, in which case the previous object model is employed -- indeed, CS4 allows you to do this for scripts all the way back to CS scripts. The Object Model Viewer in ESTK, once you've initialized it by opening it once and choosing the InDesign CS4 Object Model, allows you to see all four supported object models.
Bear in mind that as before, you can create a folder for previous version scripts and run from there, in which case the previous object model is employed -- indeed, CS4 allows you to do this for scripts all the way back to CS scripts. The Object Model Viewer in ESTK, once you've initialized it by opening it once and choosing the InDesign CS4 Object Model, allows you to see all four supported object models.