Microsoft promised Surface Dial support for the Pro 4 in Q4 2016, and we hadn't heard a whole lot since then. It's remained exclusive to new Surface devices, perhaps in an attempt to ship more units.
It's taken a while, but that's no longer the case. Surface Pro 4 users now have access to the same functionality thanks to a simple firmware update. For the unfamiliar, the dial is a cylinder Bluetooth device that can sit on the screen and be used to adjust ink color, scroll, control music, and more.
Thanks to the Pro 4's relatively small screen, you aren't going to want it sitting on the glass most of the time, but it's a useful productivity tool all the same.
Limited App Support
Unfortunately, the Surface Dial hasn't seen huge support from developers since its release, and that could be another reason for the delay. Microsoft supports it in Paint 3D and Adobe recently in Photoshop, but there are still a number of apps that could do with integration.
There's also DataMesh, which was unveiled in February. It lets the HoloLens, Surface Dial, and Surface Pen work together for a robust 3D workflow. Unfortunately, this won't be much use to most users due to the expense of the HoloLens and its experimental nature.
Still, if you're interested in the Dial, you can purchase one from the Microsoft Store. For $100, it may be a hard sell, but you may be able to pick one up cheaper in a sale sometime down the line.