DSM 4.0 is the newest version of the Synology firmware. To create a complete review of this firmware would amount to writing a book, it is that comprehensive. What's nice is that Synology's firmware is backwards compatible with earlier models. The latest version works on models starting with the 8 series, which means that DMS 4.0 runs on four-year-old models.
While Synology offers a wide array of features, there is a flipside. All these features mean that the NAS has to do more than just share files. The more functions you use at the same time and the heavier the load is of these features, the more a higher-end model can be advantageous.
The newest DSM version has a number of improvements. Perhaps the most interesting one is that related features are grouped together. For example, you don't have to install a download client if you plan on using your NAS for your business. You can now choose to only install the components that you want to use. It's still possible to add software for security cameras, an iTunes Server, a Media Server, back-up software, a mail server, a VPN server, and many other features.
If you use third-party applications, then the transition from DSM 3.2 to DSM 4.0 can cause some problems. We found this out the hard way with SABnzbd+, a Usenet client. Version 0.5.4 that worked fine with DSM 3.2, did not start in DSM 4.0. The solution was to install a later version.