Can I use my iPhone 4S as a "router" for my iPad WIFI?

If your phone carrier supports it (and you might have to contact them in case it's an extra subscription), then you should be able to use the iPhone's personal hotspot function (http://support.apple.com/kb/HT4517) to share its internet access with your iPad (the iPad should see it as a wifi network).

You can subsribe/purchase the tethering/personal hostspot plan on the iPhone 4S (if supported by yur carrier) and use that for internet connection on your iPad.

iOS: System requirements for Personal Hotspot