Continuing to build partnerships following the launch of its new Windows 8 operating system, Microsoft’s Japanese arm announced today that it has agreed a “cooperative business relationship” with NTT DoCoMo (the country’s biggest mobile operator) to push LTE-enabled Windows 8 tablets for business users in the country.

As part of the deal, Microsoft will combine Windows 8 tablets with DoCoMo’s super-fast LTE mobile service Xi (pronounced ’crossy’), leaning on device manufacturers and vendors to develop mobile products and solutions that mesh with the the operator’s LTE services and push the Windows 8 ecosystem in the process.

DoCoMo says that the collaboration will not only encompass hardware and software, it will also share its marketing and sales resources with Microsoft Japan, and vice versa.

The Japanese operator explains the partnership:

Tablet devices are being used increasingly in the corporate world to raise the efficiency and value offerings of sales activities. DOCOMO’s Xi LTE service enables corporations to use tablets with enhanced flexibility and diversity, while the new Windows 8 operating system realizes permanent connection to wireless networks and enhanced mobile experiences using the touchscreens of tablets from various manufacturers.

Earlier in the week, Microsoft announced that it had sold four million upgrades for Windows 8 in the three days since the operating system launched, noting the figure during the BUILD 2012 conference.

Steve Ballmer, the CEO of Microsoft, greeted crowds at the highly-anticipated event and said that enthusiasm for the new operating system was “very high”. The new figure is understood to include individual purchases, as well as units that have already been sold to stores and which they expect to sell.

In comparison, Apple shipped three million copies of its latest operating system for Mac, known as Mountain Lion, in just four days after its launch.