Can you use your wifi hotspot on your iPhone and make the iPhone think that it's on wifi so you can do FaceTime?
Yes. Since your own personal hotspot is techniclly wifi, if you connect an iPhone 4 to it, it will be "tricked" into letting you use Facetime.
Sadly no :(
If it's already on wifi you wouldn't need to "trick" your phone into thinking it's "in" wifi area