Hi, good news for Lenovo Thinkpad T420 users. Even though there is no official information from Lenovo about Windows 10 supports for Thinkpad T420, I can install and run Windows 10 64 bit on my Thinkpad T420. Originally, Thinkpad T420 comes with Windows 7.
The following screenshot is the Dxdiag result on my Lenovo ThinkPad T420. As you can see Windows 10 Pro is installed.
So, if you have Thinkpad T420 and want to install Windows 10 on it, you may continue with your plan. Its working like a charm.