Having used both the DX4000 and Synology Units, for ease of use and file serving, our Synology DS1512+ is hands down way easier to use and fast for serving files. We now have an expansion unit hooked up to it for 24tb of storage. It has some packages that can be run a services and has been reliable for us.
That said, the DX4000 has a full 2008 os under the hood and you can remote into it and install windows porgrams. Our time clock software and some book keeping stuff makes the 4000 a good choice as well as the computer backups. OMG why do they click the links in the email… Pretty simple to restore from backups. Can keep the sales computers on 500mb drives lol. The DX4000 has all the security and user stuff that 2008 has. Synology has some of those features (and they get better every release) but still a ways off from the 2008 server stuff. And if you have a MAC, your milage may vary with the Synology. Our video guy has a MacPro and it hates talking to the Synology. Hates it. Some days good, some days bad, but never can tell why it is one way or the other.
The synology serves files to the win boxes fast. I keep our art files there (3 and 4 gb photoshop files) and i can rip them from the server at 160 to 180 mps to my local ssd (raid zero) most of the time. The synology is set up hybrid raid (SHR) and can loose 1 disk (of 10) and stay up. The synology can also use cheap desktop drives. The DX4000, get ready to spend $200 pluson 2tb drives. And you gotta have at least 1 on stand by. FWIW.
