Replacement HDDs for RND4000 v3 - On-line list only shows Dis-continued Models

You are limited to 2 TB drives - no larger.  The WD20EFRX and the 2 TB Seagate Ironwolf are good options. 

Don't get the newer WD20EFAX - it is an SMR drive, which is not a good fit for RAID, and which could misbehave in your legacy NAS.  Also avoid desktop drives - many of those have also switched to SMR technology.

All "V3" models I know of are what we refer to here as V1.  If it's a silver box with an internal power supply, it's a V1.  Netgear did nobody any favors by re-using the V2 designation that was originally a minor hardware difference and calling an entirely new NAS a "V2".  But they never re-used "V3", so those are yet another minor mod to the V1 platform.

Why that matters is that The V1 is limited to 2TB and smaller drives.  Any NAS rated drive is going to be a good replacement, though the smaller WD Red models now use SMR technology that may give trouble if combined with other types.  They will run slower on writes of large data files, too, though I'm not sure how apparent that would be on an old NV+.

Honestly, though, I would never even consider outfiting a 10 year old NAS with all new drives -- it's slow, has an outdated operating system, and could fail soon.  And when that failure does occur, you cannot simply migrate the drives to a newer NAS, even assuming the failure does not damage the data volume.  Oh, and your NAS is also only SATA2, so newer, faster SATA3 drives will not increase the speed.