If you did things to the file that are unique in CS4 sometimes there is "invisible" information in a file that can cause a problem
Save another copy as a .tif, which is a format any version of Photoshop can open (as well as other image editors) and it will preserve layers too. That way, if the native .psd file can't be opened in CS3, you will still have the .tif layered copy that can be opened.
or in cs4
Choose File > Export.
From the File Type (Windows) or Format (Mac OS) menu, choose InDesign Markup (IDML).
You can open the saved IDML file in InDesign CS4 but not in any previous version of InDesign. To save a file for use in InDesign CS3, choose the InDesign CS3 Interchange (INX) option.