You need LR 5.6 or newer. The current version of sale is LR 6.0.
If you don’t want to upgrade then you can use the DNG Converter 8.6 or newer and create DNGs from your NEFs, a folder at a time, and those DNGs will import into the older LR 4.
Thank you so much!
The other problem you might run into is if you “touch” (transfer, view) any of the NEFs of the newer camera with Nikon software (Nikon Transfer, ViewNX) that came with an older Nikon model, that obsolete Nikon software can corrupt the NEFs and making them unrecognizable to Adobe software. This would affect either LR or the DNGC.
If you want to purchase LR6 then there are two options, the Photoshop-CC+LR6-CC (Photography Plan) for $10/month and the older serial-number-licensed version available on the Products link you’ll find on the fooder of any w w w.adobe.com page.