I just did a comparison. The Mosaic set has the following tracks not on the 4 downloads:
Mildred Bailey- Mosaic disc 3 tracks 10-13
Stuff Smith- Mosaic disc 3 tracks 14-15
Joe Sullivan- Mosaic disc 4 tracks 7-13
Count Basie- Mosaic disc 5 tracks 3,4,6,8,11,12,15,16,17,18
Count Basie- Mosaic disc 6 tracks 3,4,7,8,15,16,17
I am not positively sure about the Count Basie tracks as some of the times do not agree precisely. In any case the 2 Mosaic discs total 39 tracks and 17 are not on the downloads.
In any case the Mosaic set does include all the 4 download tracks, plus the ones mentioned above.