Since the D610 was released long after Lightroom 4 was released, there is no support for that camera in Lightroom 4. You'll need at least Lightroom 5.3 or later in order to have support for the D610. Here is the list of supported cameras, and their minimum Lightroom and Camera Raw versions:
If you choose not to upgrade, and instead want to continue using LR4, your other option is to use the free Adobe DNG converter to convert your D610 raw files into DNG format, which will then be readable by Lightroom 4. You can get the DNG converter here:
Thank you for your help.