I'm working in Photoshop in CS5 (MAC platform) and was given a request to save down an image to CS 4. Does it make any difference if this image was saved originally in CS5 if one is working in CS4?
The file from CS5 will still be compatible for CS4. You could loose some editing for the features that is only available in CS5 and up, but the file should still work in CS4.
You may want to go to preferences> file handling, and make "Mximize PSD and PSB file compatibiliy" or "maximize backwards compatibility" to "always".