Always keep a copy of the original NEF. Convert a copy to DNG for CS4, never overwrite your original raw file.

: Once converted to .DNG, the files will open in your existing CS4 Camera Raw interface without losing image quality. 4. Alternative: Nikon NX Studio Photoshop CS4 fails NEF files - Adobe Community

Here is SEO-optimized content focused on — aimed at photographers and Adobe users who are trying to open modern Nikon RAW files (NEF) in the older Adobe Photoshop CS4 (Creative Suite 4).