Does Windows Memory Diagnostics Tool scan RAM over 4GB?

This is how I went about examining the question :

I first opened the Windows 10 ISO :

And extracted the memtest.exe file, dating from 2015 (a good sign), then examined it :

To find out to my great surprise that this is only a 32-bit program. But of course it could still be using PAE to test more than 4 GB.

I then started the memory test on a VM with Windows 10 and 8 GB of RAM. The test results are well-hidden, but I found them in the Event Viewer, under Windows Logs -> System, as events which have "MemoryDiagnostics" in the Source column :

As you can see, Windows claims that the test on 8GB has passed.

So Microsoft's answer to your question is : Yes, we can. It would have been incredible had it been otherwise.