Q: How can I find my lost iPhone 4

One way is through the Find My Phone application but provided that:-

1) the iPhone is connected to the internet (be it by way of Wifi OR data); and

2) you have previously activated the feature.

Then again, even if the above conditions are true, you will most probably only get the rough location of the phone and not the "exact" location of it.For instance, if the iPhone is in a high rise building with 60 floors, you will only know it is in the building but never which floor it is.

Unless you are carrying your laptop all around, activating the Siren would assist you in finding your iPhone.

Good luck (Taken style)