I just installed eight (8) 4TB Seagate NAS drives in a Synology DS1813+. The original drives were 2.5" 1TB HGST 32MB 7200RPM drives for low noise, energy efficiency, low heat. The Seagate NAS drives are just as quiet running. There is a distinct spin up and seek is only audible literally with ear right next to the DS1813+. Temps have since stabilized after building the array and optimizing the file system. Amazingly, it about +4C over the previous setup. Oddly, the iSCSI Block-Level Target runs at the same temp in use as the 2.5". This leads me to believe the temp increase is more due to restricted air flow of 3.5" vs 2.5" drives. The drives are fast enough to saturate gigabit at +120MB/s for sustained transfers, both single drive (iSCSI) and RAID 6 (AFP & SMB) configurations. The drives have been running since Friday, so I can't comment on reliability.
WD Reds are not available in 4TB, so I ruled them out. WD SE drives appear to be the fastest NAS option at 4TB, but are $70-80 more than Seagate NAS because they are Enterprise drives like Seagate Constellation.