The fourth volume in Naxos series of Sibelius' piano music under Håvard Gimse presents several collections of miniatures from the late middle of Sibelius' career. This is a variegated and variable set - several of the pieces are frankly no more than mediocre salon pieces that no one would want to hear again. By far the most interesting works are the op. 75 and op. 76 sets, both of which contain several small gems. Sibelius' piano music is in no way on the level of his orchestral works - and neither do they sound anything like them - so people coming to these pieces expecting that will be disappointed, but that shouldn't stop anyone knowing what to expect from enjoying the best of these pieces in their own right.
Gimse plays them excellently (I have not heard Mustonen's recording of most of the pieces here, so I cannot compare) and the sound on this volume is excellent. Sibelius' piano music has received mostly disparaging remarks over the years, and it is true that his piano music doesn't hold a candle to his orchestral works. Yet it is often not bad music, and the adventuresome does well to investigate. That said, I would probably not start with this release - the quality is a little too variable for comfort, and only the named op. 75 and 76 sets are really worthwhile - but rather with vol. 1 or vol. 3.