Upgrades over the 4900:stem, saddle and rear derailleur
Upgrades over 3900: Derailleur, Fork with Lockout, Frame and wheels.
Upgrades over the 6300: Fork, Wheels, Crankset, Stem and Seatpost
Upgrades over the 8000: Wheels, fork and brakeset.
Upgrades over the 4300: Fork, Gears, Wheels, Hydraulic Disc.