A new restoration of the 1938 Fourth with the same forces was just posted as well, with this explanation of the process:
A mono recording does not have discreet multichannel information, but it has the same field acoustics as stereo. This is called the Field Characteristic. Microphones react to sonic energy the same way our ears do. We don't hear in stereo, we hear something much better: field displacement. So do microphones. The process used here detects this field displacement, which is fed into an unpanned six channel matrix based upon the original temporal scaling, which is also preserved in every mono recording. The result is very similar to stereo because it contains the same acoustic information as a stereo recording without the use of multichannel. What you are hearing are the original acoustics as the microphone picked them up; the same thing you would have heard them during the recording session itself.
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FYI, if you're a fan of this performance, a beautiful new restoration was posted on the YouTube "Private Reserve" channel today.
There was also a restoration of the 1929 Clemens Krauss performance on the same channel two months ago.
Two amazing interpertrations, IMHO. A critical review summary can be found here
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