How to download hp4520 driver on my iMac ipad

If that printer is a Wifi/AirPrint compatible printer, you need to check to see if it is compatible with iOS AirPrint and if it is you may need to install an HP App from the iOS App Store.

If this printer is NOT a WiFi/AirPrint ready printer, then there are WiFi network printing apps that you can install from the iOS App Store.

If you own a computer that is connected to your local WiFi network, these types of generic WiFi printing apps require you to install a companion app, located from the printing app’s developer website, that you have to keep active so, the print activity goes from the Wifi printing app on your iPad, to the WiFi companion app on your computer to process your print request through the normal printer driver on computer that will initiate the printing to your computer connected printer.

Good Luck to You!

You don't download drivers on an iPad, you cannot install drivers on iOS devices.