I don't have a solution but I experienced the same problem on my Laserwriter 4/600ps.
Random postscript errors were popping up etc.
My suspicion is that it has to do with the more recent Laserwriter drivers, as the printer *used* to work just fine under... oh, like System 7.6.1.
On the other hand, stuff I printed back then may have been simpler to digest.
One tip: makes sure the proper PPD for the 4/600ps is selected in the Laserwriter setup dialog in the Chooser. Might make a difference.
This error-proneness is a nuisance, especially, since there are few PS laser printers in the 4/600s price/quality range nowadays. LaserJet II emulation under PowerPrint just don't cut it.
HTH,
Andreas