I bought the 4870 just before the 4990 hit the American market (and got it for a song). It does a great job on 120 & 4x5 film. Certainly good enough for most uses and anything that requires drum scanning can be outsourced. It works just peachy for 35mm minimal enlargements (snap shot prints) too. So I would imagine that the newer model will do just as good a job or better but for my coin, the 4870 won.
Good luck in your choice