My take here is a little different from that of other reviewers. Whereas Mahler may have clarified the mud at certain points (and as the notes to this recording say, Mahler curiously didn't even touch the doublings of wind and strings that are thought of as Schumann's most egregious failing in reworking his Symphony No. 4 , 1841/1852), at others Mahler misjudges, I think you'll agree, if you know and love these scores well enough. For example, he makes a strange cut in the coda of No. 2 just where Schumann builds to a big Romantic peroration, and there are a Brucknerian luftpause or two thrown in for bad measure. These will leave knowledgeable listeners scratching their heads. But otherwise, the changes are subtle and if not essential, then mostly for the good. Woodwinds make their points more tellingly here and there, and balances in general sound more judicious.
(It's a shame, by the way, that while he was at it, Mahler didn't cobble together the truly definitive Schumann Fourth Symphony. This would start with Schumann's 1841 edition of the symphony, preferred by Brahms and just about everybody else who has really listened to the two versions, and for the two big transitions in the work--slow introduction to First Movement proper and Scherzo to Finale--substitute the more successful versions of these sections from the 1852 edition. Schumann learned a lot about building tension in an orchestral piece in the years between 1841 and 1852. But during the same period, he also became a beleaguered conductor who, in order to get more surefire attacks from his orchestra, decided to double the winds and strings in his later edition of the symphony. The result? The listener strains for evidence that the woodwinds have even been invited to the party.)
The trouble with Mahler's interventions, beside the fact that they represent questionable musical practice in the first place, is that the authentic-music movement has shown that the original orchestrations were not as inept as many thought. It's just that Schumann's scoring doesn't work well when played by the late-Romantic orchestra that we hear in concert halls of today. Unless, of course, a great late-Romantic composer comes along and makes the proper adjustments to the Schumann sound so that it all does work. John Elliott Gardiner, in his essential recording of the complete symphonies, went the other route, of course, stripping the orchestra down to the Mendelssohn-era one that Schumann would have known. And, of course, the original orchestrations work much, much better, even if Schumann won't ever be considered a master orchestrator.
So what of the current recording by Chailly and the Gewandhaus Orchestra? Two things need be said. One, it is fascinating to hear Mahler's thoughts on music he clearly loved and felt he had to "improve" before bringing it before the public in his guise as conductor. Two, and more importantly, these are brilliant performances in any event. Chailly is one of the two or three consistently fine conductors recording today, and the Gewandhaus (Mendelssohn's orchestra, after all) sounds like a hand-in-glove fit for this assignment. A case in point is the "Genoveva Overture." Mahler didn't tidy up this work for Schumann, and yet it makes as grand an impression as the symphonies do--maybe grander, in that magnificently brassy peroration at the end of the coda. I've never heard it more thrillingly played, and this is one of my favorite Schumann pieces.