Q: how to use wifi in iphone 4

Hi, Ibrahimseu20d. 

Thank you for visiting Apple Support Communities. 

If you are wanting to connect to a Wi-Fi network, here is a resource I would recommend. 

iOS: Understanding subscription Wi-Fi networks

http://support.apple.com/kb/HT3867

If you are wanting to use your iPhone as a personal Wi-Fi hotspot, I would recommend this article. 

iOS: Understanding Personal Hotspot

http://support.apple.com/kb/HT4517

Cheers,

Jason H.