Clueless I started Google search for some hint about hard drive replacement but I wasn't successfull no matter how hard I've tried. No luck, I had to find out on my own. Looking at the frame, no single screw was visible. The only thing which could be removed was the battery. Once detached, four screws were appeared which looked promising. After unscrewing nothing happened, nothing fell apart, almost no part became loose. At this point I want to tell you how to continue without potential damage. Here it is:
Remove the top grill with power switch: actually removing four screws in battery compartment allows you to detach the grill which let you continue with disassembly. When unscrewed, turn notebook on its stands and open the lid. Gently push and slide the grill away from you. You can press both rubber bars with left and right thumb and slide forward. Now the grill should release and you can remove it. You immediately get access to another four screws holding the keyboard.
Remove the keyboard: after unscrewing four screws from above step you continue with the same trick, slide the keyboard away from you, do not pick it up now. After sliding it will get loose and you can remove it. Beware keyboard is still connected with the body via ribbon cable. It is possible to disconnect the cable from the plug and continue keep it away. Actual hard drive is hidden underneath of the palm rest which you have to remove in next step. If you want just upgrade the main memory, you can stop reading now. You should see memory modules once the keyboard is removed.
Remove the palm rest: this step is very easy since instructions are engraved directly next to left screw you have to unmout. After unscrewing you continue with sliding the palm rest to right ! Now you can see hard drive on the right side in metal cage. The cage can be released unscrewing three screws and sliding the drive right. Drive replacement procedure is now the same as with any other notebook computer.
I hope these steps will help anybody of you to successfully upgrade your ProBook 4320s hard drive ! Good luck.