The iP4500 and iP4300 printers are very similar, so this druckerchannel.de instruction for disassembling the iP4300 for replacing the ink absorbers does also apply to the iP4500. If you don't read German, then this Google translation might be helpful, even if it is less than perfect.
The service manual might also be helpful, here is a simplified service manual with a parts catalog added. Be aware that there is an error in the service manual regarding service mode operations. Please read this warning before using service mode if you are not familiar with service mode operations on your printer.