Is it possible for me to use my iPhone4 microsim dataplan on an iPad2?
No, not since 4.2 can you do this because the APN settings disappeared off the devices.  

Though, there are videos on youtube and tutorials in other places that can walk you through adding the features back on to your device. Whether or not they still work or will work for you depending on your current iOS version installed on iPad is a different story. 

A particular Video here:

http://www.youtube.com/watch?v=CUnPQ68DNO8

If you read the comments though, it doesn't work for everyone, and some people it did work for had issues with their sim cards after about a week of use, with AT&amp;T disabling them.

Other people on here might tell you to jailbreak your device and enable tethering but that is your choice.  Jailbreaking isn't for everyone. 

A website here talks about making a wifi hotspot with your iPhone the legal or jailbroken way and talks about the pros and cons of each:

http://www.gadgetreview.com/2011/09/how-to-turn-your-iphone-into-a-wifi-hotspot.html

I hope that this answer helps you out.