When I wanted to confirm my understanding of the same issue going from CS2 to CS3, AE expert Mylenium provided some useful details about this subject on the Adobe forum …
Mylenium – 10:45pm Feb 5, 09 PST (#2 of 4)
As Navarro said: Should be no problem. Be aware, though, that several of the “Edit original” and “Open in Bridge” type functions will always resort to the most modern version and these functions thus will not work fully in older versions. If e.g. you chose to “Browse Presets” in AE CS3 with CS4 already on your system, it would open Bridge CS4 and you would be able to view the presets, but could not apply them with a double-click as you would otherwise. Also expect some confusion when working with MPEG stuff. You may get warnings about missing CoDecs in Encore and Premiere or weird licensing warnings (“not included in trial version” and such…)