hi steve, thanks for the response.
mamiya 645 pro with mirror up
very large studio stand
subject is large and wouldnt move
have you used a eyelike back?
I haven't shot with a Mamiya Pro, but your overall setup sounds viable. Since you bought the back used, it's possible it's out of calibration. But you said 4 shot is right on the money, so I don't know. 16 shot can be a very trying experience in that regard. When you say 16 shot is not right, what do you mean? Are you seeing cross hair patterns along edges, or ??
I am a dealer for Eyelike (as well as other backs - Leaf/Hasselblad/Sinar) and have primary experience with the M-series (firewire-based). Because you indicated 4 shot was sharp, it doesn't sound like an out of calibration issue. However, Sinar now handles the Eyelike - product in the US at least - you could contact them for re-calibration.
Another potential solution. I could contact one of my Eyelike users and see if they would allow you to borrow their calibration chart and go through the process just to eliminate that possibility. Contact me offline if you like. Again, because 4 shot is dead on, I don't suspect a calibration issue.