You just need to install rollers yourself. Below is a link to one place to get them. Keep in mind the way this printer works is if the tray has the same size and type of paper in the trays, the printer will pick from the high number trays but lower physically on the printer so the if the bottom trays has the same size and type as some of the trays above, the printer will always grab from that tray as long as it has paper and that trays rollers tend to wear first. Also each tray position has 2 rollers. One in the tray which is a separation roller responsible for insuring only on sheet is fed at a time. Inside the cavity is the same type roller which move the paper forward to be fed. There are also 1/2 moon rollers in each tray cavity, but to be honest they rarely wear. If you printer has over 1 mill copies then may they should be replaced. Do a google search for the service manual which is free on many sites for download as replacing the rollers is very easy. Another issue which can cause jams is the backfeed clutch that sits in the tray that the roller attaches. To fix that problem you just remove the clutch and hit the small shaft on a hard surface so the clutch separates. You then remove about 1/2 the magnetic filing on the round piece, reinstall and it will be good a new.
http://www.ebay.com/itm/HP-Laserjet-4000-4050-4000n-4050n-Paper-Jam-Roller-Repair-Kit-/321469091956?pt=LH_DefaultDomain_0&hash=item4ad90d1074
http://www.ebay.com/itm/HP-Laserjet-4000-4050-4000n-4050n-Paper-Jam-Roller-Repair-Kit-/321469091956?pt=LH_DefaultDomain_0&hash=item4ad90d1074