If you need 4K/1080p transcoding, consider the QNAP TVS-872XT.
But bear in mind that only the QNAP apps work over the HDMI port. You can run Plex Media Server on this NAS and it supports hardware-accelerated transcoding up to 1080p. But there is a PCIe slot allows you to extend the NAS capabilities by adding 10GbE support, SSD caching, or even a wireless card. There is only 1 Gigabit ethernet port, so link aggregation is not possible out of the box. This is great for running different operating systems on your NAS, such as testing/development on Android. The TS-251D also supports virtualization, so you can run virtual machines. Also the new processors have the Intel HD Graphics 600 on-board which offloads certain tasks from the CPU. But actually the J4005/J4025 are much faster than the outgoing J1900. You may think having 2 cores less would handicap the NAS. The aging quad-core Celeron J1900 processor of the TS-251+ has been replaced by the new dual-core Celeron J4005 (J4025 in some countries). If you want a powerful Intel chip in your NAS, the x51 series are the go-to QNAP models. Moving into QNAP’s Home and SOHO product offerings, the TS-251D improves upon the popular TS-251+ model. Also the limited RAM and processor allow only for software transcoding at 720p. So factor this into the total cost of ownership. Only 2 free IP camera channels are included though, and extra channels cost around $50 each.
Linux doesn’t need as much RAM as Windows and so 512MB RAM is good enough for recording up to 12 IP cameras at 1080p and 10fps using the built-in Surveillance Station app. While the 512MB RAM may not sound like much, bear in mind that these NAS boxes are basically extremely energy efficient Linux PCs.
But one big upgrade is the new quad-core processor, which improves performance substantially. This NAS looks very similar to its predecessor, the DS218j.
The DS220j can serve as your file server, stream media across the home, automatically backup your devices, set up your own private cloud for file-sharing and monitor your IP cameras as well. The Synology X20j series is the starter option in the QNAP NAS family and range from 1-bay to 4-bay models. Synology is well-known for its high-quality hardware and very intuitive OS called DSM.