There is lots of really nice additions to LR 4. The new processing is superb (more later). One nice addition that was done is that we can now individually manipulate the red, green, and blue channel curves. So I played with them a little last night and generated a few presets that sort of simulate cross processing, as well as a set of film stock. I glanced most of the ideas on this from the web where I found lots of advice on how to do this with the same curves tools in Photoshop. Below is an example of their effect using a silly outdoor picture of my son at the train museum, a sunset over the Golden Gate, and a sunset at Yosemite (click for bigger)
These are a little over the top (they were made late last night) but illustrate the concept well. If you want to try for yourself, download a zip file I made of them here. You should be able to double click them to install them into LR 4 or you might have to import them in the Lightroom develop module.