The D750 requires at least Lightroom 5.7. (which is no longer sold)
You can upgrade to Lightroom 6 here: Software & Services
A free alternative is the Adobe DNG Converter, which will convert the NEF files from the D750 to DNG files that can be read by LR4.
But you will be missing out on all the new features in Lightroom that have been introduced in versions 5 and 6.
Another alternative is to subscribe to the CC Photography Plan - Lightroom and Photoshop for $10 a month.