Q: I can't see airDrop feature in my iOS deviec.

unfortunately iPhone 4s does not support that feature. Ipad mini, iPhone 5 or higher, iPod touch 5th gen and ipad 4 support it

Airdrop can be found in many places.  If you swipe up from the bottom, you should seen it in "Control Center".  Another option might be to go into "Photos", select and image and click the lower left button.  You can choose it to be sent to another iPhone via Airdrop.  These are just two examples but are you looking to do something specific?

...Please forget what I said.

- JC

Not on any Apple site

see this

http://support.apple.com/kb/HT5887