If the paper reaches the output bin but is wavey or wrinkled, then either is an issue with the fuser unit, or a common problem is with the paper output assembly which is Part# RG5-0886 and is about $57. You can buy a repair kit instead of buying the entire assembly which work good but may not be for you if your not good at performing the task which is Part# K0456. You can get a fuser on exchange for $39.00 which is Part# RG5-0454E. If you decide to replace the fuser I would suggest ordering a Maintenance Kit (fuser exchange) instead which should be changed @ 200,000 pages. If you print a self test or config page it will show the page count. The maintenance kit includes pickup rollers for the manual tray, PC tray and the optional LC tray, comes with the fuser (on exchange), transfer roller, manual tray seperation pad. It doensn't come with the output assembly. The output assembly on the HP LJ 4 was engineered wrong by HP. The new replacements are slightly modified.
All the parts are from TSA. Phone number is 1-800-633-6626. These prices may vary slightly but should change much when you order.