how to scan a barcode on an airline boarding pass on iPhone 4s

Try the QRReader app from the app store...it reads bar codes and the QR codes used for boarding passes.

Do you mean you have an electronic boarding pass on your iPhone that you wonder how they airport will scan it? If so, you just open the boarding pass (usually sent as an email or an email URL) so the pass is showing on the screen and hand it to the airport security person. The scan it under the same scanner they use for printed boarding passes.

If you are asking about the mechanics of using an iPhone, or smartphone, boarding pass, after doing this recently, in the US each family member traveling together must have their own iPhone with the barcoded boarding pass for TSA acceptance. That is, two family members on the same ticket purchase cannot use one cell phone for both. Even though the airlines gate attendants can read the data as applying to two passengers, the TSA security point will not accept that. Each must have their own, same as if they were using paper boarding passes.

You can also use the Boarding Pass Scanner application which decodes the content of your boarding pass - It's on iTunes at https://itunes.apple.com/us/app/boarding-pass-scanner/id820796885