The 4005 is the new model that replaced the 990 and basically is the same exact tractor. It comes standard in 2WD but you can get it in MFWD as well.
I just purchased a used 2005, 990 with 174 hours on it. It has the 41.5 hp diesel engine and it has lots of power. It has a 300CX FEL, the JD 8B BH, JD Pallet Fork and the JD Ballast.