My experience may or may not apply: Our original 4900 was replaced with a refurb machine after a head failure, Our original machine never showed paper path problems. The refurb's cassette path quickly showed trouble, with sheets either never leaving the tray during a print cycle, or sheets being picked up from the tray but jamming the path early after making the hairpin bend. Replacing the pickup roller made no difference; ultimately a new paper cassette did solve the problem.
What was happening, we learned, was that the cassette "rear hinged door" was hanging and would either never drop down (like a draw bridge does) or it would "reluctantly" drop only after deforming the leading edge of the sheets. It was the sheet deformation that caused jams; and when the door never fell at all, the paper of course never entered the path.
Short of replacing the your cassette tray, look at the cassette's rear door and see if there are any observable reasons that door would not fall. Also shine a flashlight into to rear of cassette (with it in place and paper loaded) during a print cycle, and watch the behavior of the pickup roller and the cassette door. There may be some diagnostic clues there.
Good luck,
John Caldwell