Most users that use a Mac for pro applications usually turn to the Mac Pro as their computer of choice because of its computing prowess. The Mac Pro is a high-end Mac with a high-end price, but it is not the most powerful Mac available on the market. In fact, the most powerful Mac isn't even sold by Apple, it is actually sold by Big Little Frank, a company based in Denmark, which makes custom Macs.
There is a small community of creative professionals who are taking their 2009-2012 Mac Pros and modifying them to be even more powerful than the computers Apple sells today. The older Mac Pros can use top-of-the-line graphics cards, which are not compatible with the newer Macs, allowing them to crush the benchmarks, even when compared to newer computers.
The Mac Pro 4,1 and 5,1 are the models which are being used because they are the last of the tower Mac Pros which could be modified, before the "trash can" design Mac Pro came out. There is a Facebook Group, Mac Pro Upgrade, where you will find people buying these old Mac Pros off of eBay and Craigslist and then modify them to be a beast of a computer.
Similar to the Hackintosh community, the community on the Facebook group has found solutions to many of the problems they have faced and even created a list of compatible and incompatible parts. There are also written and video tutorials to help with upgrading the computers. Some have also found a way to upgrade the Mac Pro 4,1 to the 5,1 with a firmware update.
Big Little Frank is aimed at pleasing the "pro" community that needs a Mac that is even more powerful than the one sold by Apple today. They just like many people in the pro world are anxiously waiting for Apple to announce the new Mac Pros.