The 4500 is an old unit, that is at it's EOL.
If you have used new toners, preferably HP, then the problem lies with the carousel. Check the rotary drive for wear, and blow out the sensor on the top right of it.
H\V contacts may be dirty, the developer PCA may be defective, or you have cheap toners that are blown.
The cost of all this? Buy a refurbed 4550! The key to color printing is keep it clean!!!!