if i buy the new iPad, can i switch between wifi and 4g service , say when i am in a wifi hotspot, or if i want security [do banking etc] or if i want to save my data rate.

if yes, then how?

Like an iPhone when you are connected to wifi you don't use the cell data. FYI if you connect to wifi you are not secure to do your banking if you connecting to a source which is not your own

I would imagine it'd be as simple as turning off the wifi. If the wifi is off, then the cell service kicks on.

You do need to enable Cellular Data, at least on the iPad. I've understood that, when connected to WiFi, cellular data will not be used. Although I wonder at times when there has been a weak WiFi signal. My usage of cellular data went up pretty quick.

For the new iPad, there is a item called "Mobile Data" in the settings app. Select that, and you can turn the cellular data on or off as you choose.

As Joker Eh said, be wary of using public Wi-Fi hotspots for any security related activity, like viewing your banking information. Better to wait until you get home or use cell data for that, as it is secure.

You can turn Cellular data on or off from the settings app. I second the notion that you should not do any private work in a Public hot spot, due to security issues.