Digital BW, The Print

Let's tackle the easy part first... The 4000 houses 1 matte black 
cart, 1 photo black cart, and 1 light black cart all at the same 
time. So in reality, you already have 3 blacks to use on matte 
papers, and 2 on gloss. If 2 inks is enough, then you are all set. 
I'm sure that most will recommend using Eboni for the matte black, 
and whatever matches for the light black (the Epson light black tends 
to be a little warm). To get good results, try Quadtone RIP running 
on a Mac OSX or PC.

Now the hard part. Many high end RIPs will allow you to put nearly 
any color ink, in any channel, and the good ones let you control 
mixing between channels. You may need very expensive and complex 
profile creation tools to make these multichannel profiles (like 
Monaco Profiler Platinum) to make it work. Expect to pay a lot of 
money for this combination. It would be cheaper to buy a 7600 for 
color, and another 7600 for dedicated grayscale work. I suggest you 
look into option 1 first. And if you are going to buy something 
refurbed, think about a 7600, they are not much bigger than the 4000, 
but only hold one full black at a time, and the light black.

The refurbed 7500s seem like a rip off to me, and I question how much 
is really refurbished, and how much just has the counter reset. If 
you bought one from Epson, then it might be OK, but from other 
places, I question how thoroughly they really get checked. A refurbed 
machine should have new heads, and motors, as well as all other 
maint. items replaced. That's about 8 hours labor and adjustment plus 
the cost of the parts, so I find it hard to believe that everything 
really gets done.