i lost my iphone 4, how can i find it?

If you turned on Find my iPhone, go to http://icloud.com, and click on Find my iPhone. If you didn't, you can't track it.

The only way to find a lost iOS device is to use the Find my iPhone app or www.icloud.com. For this to work, you have to enable this feature on the device prior to loss. If you didn't do this, there is no way to find it. Niether Apple nor your carrier will find it for you.

Setup Find my iPhone:

http://support.apple.com/kb/PH2697

Here's a good tip from Kappy on what to do if you device is lost or stolen:

https://discussions.apple.com/docs/DOC-3362