Can I use my iPhone 4S to take my passport photo?

It's an 8 mega pixel camera.

In the United States, passport pictures need to be taken by an authorized passport photo agency.
You find them all over… Even the post office.
I did mine at FedEx Office.

You can, but you need to make sure you shoot your picture in front of a white background, and when you get the picture printed, it needs to be a specific proportion.

IMO, it's not worth saving the $10. Just get it done where they do that all the time.

No. To get a passport photo, you need to get it taken properly at a place that specializes in passport photos. Most drug stores like CVS, Walgreens or Rite Aid have people there who will take passport photos for you.

No, you need the stamp of the place & location that did it for you.