Hi, Charles, and welcome to the forum.
I'm assuming you want to use the personal hotspot feature of the 4S. There are not really too many other reasons to link the iPad and iPhone through Bluetooth.
If this is so, turn on Personal Hotspot in Settings on the iPhone, then ensure that Bluetooth is enabled on both devices. The personal hotspot should take care of the Bluetooth link. As far as I can make out, it isn't possible to make the link manually. (I'm trying this out right now). If your phone is connected to wifi, the personal hotspot will not work, but since your iPad will also connect to the same wifi it isn't necessary.
If I've missed your intentions and you need this link for another purpose, please let us know so we can try to help with this.