The 4/600PS is based on the Canon PX engine. Slow, but rarely has problems. Toner and parts are dirt cheap as HP sold PX engine printers as the Laserjet 4L/4P.  The "slow" 4PPM printing was a compromise to keep costs down. It was , after all, the first "personal" laser printer out there! I happen to still occasionally use this printer's cousin, the Laserjet 4MP. Still works great and can't beat the $15-20 toner carts. Also the HP version takes standard SIMMs, so mine is expanded to a ridiculous 22MB.