If you enter your product number from your ProBook 4400 into the HP Parts Store you should be able to find the part numbers for the keyboard and touchpad assembly. You can use these part numbers to see if they are available on the parts store. You can also provide the number to a local repair shop if you plan on having them replace it.
If your plan is to replace them yourself you can use eBay or Amazon to search for the part numbers and price.
In order to replace the keyboard and touchpad, you can enter the product number for the Probook into the HP Support Center and use the Maintenance and Service guide available for your product page. On your product page, it may be listed under Manuals or User Guides.
According to the steps in the Maintenance and Service Guide and your level of comfort disassembling the laptop, the time will vary, but I've done replacement on similar notebooks in under 30 minutes.