Although your iPhone4 microSIM will physically fit in to your iPad it's not certain that you'd be able to use it without incurring additional charges. Some carriers do not mind if SIM cards are swapped between devices while others do. So, without knowing your carrier's policy on this issue, it's impossible to say. When you receive your iPad simply try placing the card from your iPhone into the iPad. Initially there won't be an issue - i.e. you'll see the signal bars indicating that you're receiving a signal. But, when you first open Safari, it may direct you to your service provider's website and ask you to sign up for a new account. If it doesn't, then you're OK.
In the UK I have SIM cards from three service providers. Two of them I can move freely between my devices and there's no problem, whereas the third always takes me to the service provider's website if I place it into a new device.
Tim
