The D750 is not supported in LR 4, you need at least version 5.7.
Your options are either to upgrade to LR 6 ($79), or subscribe to the Creative Cloud Photography Plan ($10 a month) which includes Photoshop and Lightroom, or use the free DNG converter to convert your raw files to DNG files that can be read by Lightroom 4.
Thank you. Figured out I did in fact purchase version 5 two years ago, bit
my wife reloaded 4. Do I now need to just uninstall 4, then try and
reinstall 5? Thanks!!
It's your choice whether or not to uninstall Lightroom 4. The two programs are completely separate. But you probably don't need Lightroom 4. You could just install Lightroom 5 .7 .1 and go from there.
I appreciate your time!!