I just tried printing to full sized Super B sheets for the first time since upgrading to LR4 and the printer is printing past the end of the page. In other words it ignores the right margin when using Landscape orientation.
This does not happen when printing to Letter (8.5x11) or US B (11x17) only Super B (13x19). In all cases the margins are shown correctly in the Print Module window.
I am running on MacOS 10.6.8, and with the latest Epson driver 8.67.
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I tried an experiment and doubled the width of my right margin in LR from .13 inches to .26 inches and got it to print with a correct margin.
I also tried printing the same image from Photoshop CS5. And there in the print dialog the preview shows no margin on the right side but normal margins on top, bottom and left. However, if I change to a different paper size the preview will show a margin on the right.