QTR-Quadtone RIP

Hello,  I am new to quad tone printing.  I have done a fair amount of color printing.  I have an Epson R1800 and I will be using the MIS "black only" inks.  I have ordered them, and I am waiting for them to arrive.

I wanted to try out QTR and see how to make it work.

I have 2 questions.  When I set it up for the R1800 printer, and the R1800-3MK, and the 3MK Premier Art Smooth paper (actually I just have the regular inks in the machine, and just a plain piece of paper), I get an error saying: "Image ToRaster encountered a problem".  I tried using "Debug" enabled, and I got no info in the debug windows.  What am I doing wrong?

The picture is getting printed and it looks OK considering that only 1 of the 3 locations has the black ink.

Another question:  If I am printing with 3 identical inks (they are identical, right?) then why do I need 3 of them, and not just 1?  If 3 are better than 1, why not 4?

-Tony