Support for the Nikon D750 was introduced in the latest version of LR 5.7 and ACR 8.7 on Novemder 18th 2014.
Further updates to LR 4 were stopped when LR 5 was released on June 9th 2013. No further updates for bug fixes and new camera support.
Nada, LR 4 will never support Nikon D750. The Nikon D750 was introduced into the market in September 2014 some 15 months after further development of LR 4 was discontinued.
You can use the Adobe DNG program (free download for the package) to convert the Nef (raw) files from your Nikon D750 to the Adobe DNG format which will permit you to import those into LR 4. This is the crutch provided by Adobe to allow for the processing of raw files with outdated versions of LR and ACR.
You can also update the ACR plugin for PS CS6 to version 8.7 which can also work with the raw files from the D750. For direct support in Lightroom you will need to upgrade (paid) to version 5.7.
LR 4 will never support D750 NEFs because Adobe does not update camera support in versions older than the current one and we are on LR 5 now. PSCS 6 can be updated with ACR 8.7 which supports the D750 (as does LR 5.7).
TThanks for your answer I wanted to mak both replies as correct.That doesn't seem to be possible. Cheers
TThanks for your help.