Regarding the upcoming box set from BR Klassik, this much is given on the Amazon page:
"In his nine symphonies, Gustav Mahler created an entire world for himself and his listeners. More than any other composer, he tries in his symphonic oeuvre to get to the very depths of the circle of life the eternal cycle of growth and decay. What set of complete works would be more appropriate, therefore, for bringing the qualities and the unique sound of one of todays leading orchestras to their fullest expression? In the Mahler complete edition released by BR-KLASSIK, the Symphonieorchester des Bayerischen Rundfunks under some of its most influential conductors, but above all its current chief conductor Mariss Jansons tackles Gustav Mahler's symphonic works. The performances were recorded live between 1996 and 2016 in the Philharmonie im Gasteig and the Herkulessaal of the Munich Residenz, and most of them are recent. Yannick Nézet-Séguin conducts Mahler's Symphony No. 1, Mariss Janson conducts Symphonies Nos 2, 5, 7 and 9, Bernard Haitink Nos. 3 and 4, Daniel Harding No. 6 and Colin Davis no. 8, the ""Symphony of a Thousand"". Numerous soloists and choirs are also featured. The recordings form a challenging part of the symphonic repertoire from the recent history of the Symphonieorchester des Bayerischen Rundfunks; under the baton of Mariss Jansons, and other conductors of the late 20th and early 21st century especially associated with it, this renowned orchestra, with the musical and interpretive mastery for which it is justly famed, provides truly memorable performances"
If they're using Colin Davis for M8, one wonders why they didn't also use his really fine M4. Both were previously released on RCA. That said, the Haitink M4 hasn't been released separately.