According to the manufacturer's specifications (http://h20564.www2.hp.com/hpsc...), these RAM bars would fit. However, since a 4GB bar is already installed, it is not worth buying 2x 4GB. I would recommend the following product:
Corsair ValueSelect (1x, 4GB, DDR3-1333, SODIMM 204)

Otherwise, you do nothing other than remove a bar in order to install an equivalent one...

PS: With this device, however, you should consider whether only a RAM update will really bring the desired result or whether you would rather buy a new device. This device already has a few years under its belt.