One of the biggest changes from CS3 to CS4 is moving from adjustment layers to the new tone and color correction workflow. Once you get the hang of the features of the new adjustments panel, you will find it's faster and more convenient than the old method.
In the new CS4, there are some minor improvements to the Photomerge function, including the new collage option. Jay Kinghorn goes over the different ways of bringing your images into photomerge, the layout options, and some quick tips for creating better quality panoramas.