Hi,
Click the Wireless button on the printer, then go to Wireless and seelct Restore Network Defaults.
Nect click the Wrench, select Tools and select Restore Factory Defautls.
Next turn the printer Off and On again.
From your computer go to Devices and Printers and remove any instance of teh 4620 by selectign Remove Device.
Reboot your PC and open teh HP Officejet 4620 software.
Select Connect a new printer and follow the steps by selecting Wireless connection, you will be provided with the steps to connect it to your new network.
Shlomi