[eye4clr]

My output profile was generated through Eye One Match using the 260 inkjet option.

I think you've got it. Excess GCR will posterize 3/4 tones. Blue to black is the most common problem.

Yes, inkjet 400 will take you from GCR4 to GCR3 (on a scale of 1-4). I think it also moves the black 'start' much further to something like 70.

Just be sure you've got precise ink restrictions since the icc will then allow 400% total ink.