A lot has been written about the supposed superiority of the K3 ultrachrome printers (4800/9800/7800) as opposed to the previous generation (4000/7600/9600). There seems to be general agreement that there is no significant difference when printing onto matte media. This makes sense as the matte black ink is the same between the two generations of printers and none of the other supposed advantages are likely to impact here. What I see with semi-gloss papers (high gloss papers are still not suitable even with K3), is slightly less gloss differential and less bronzing. Both of these issues can be helped with spray such as Lyson Print Guard (there are other similar products). I really do not think there is enough difference to justify swapping from 'K2' to K3 on the basis of print quality; provided you have good profiles, the print quality is almost the same. I know there have been claims of better ink consumption, but I am sceptical about this and the data applies only to matte media. No one I have spoken to in the industry supports the economy claim.
Phatte black seems to me to be returning the 4800 to a 'K2' state and when you add on the cost of IP, this just does not make sense, especially if you have got rid of a 4000 to do it. I would be inclined maybe to get a second 4800, particularly as the 4800 does seem to do mono better straight out of the box (however, again profiles are really the key to this).