can I select the 4.2ghz radio on by new Ipad?

Your iPad does not directly provide means by which to select use of 2.4GHz or 5GHz WiFi. If your WiFi network has support for both bands - and both use the same WiFi Network Name / SSID - your iPad will automatically select last selected band when connecting.

In practice, you can force your iPad to use either the 2.4GHz or 5GHz band by configuring your WiFi Router to use a different WiFi Network Name for each band (if supported). In so doing you’ll be able to manually choose the WiFi network that you prefer. If both networks are configured in iPad settings, the iPad will reconnect to the last used WiFi network - and remain connected unless the chosen network signal performance is poor or has low signal, at which point the iPad will switch to a better performing network.

No. iPads support 2.4Ghz and 5Ghz only.

There are no home networks, or public Wi-Fi networks that use that frequency.