Tap to focus/zoom in Camera
The iPhone 4's new 5 megapixel camera has a fantastic digital zoom. But to activate it you first need to focus on an object – like a face, for example, in your view. Do this by simply tapping anywhere on the screen. A white square will appear showing you the object that the iPhone is focused on. A zoom bar will also appear, which you can use to zoom into the object you've picked.
Where’s Facetime?
This is probably one of the first thing you’d like to explore on new iPhone 4, but where’s the Facetime app? Facetime is actually not an application, in fact, it’s an option available (for now) only when you make a call via WiFi to another iPhone 4 user.
Multitasking: update your apps
Multitasking is one of the big new features of iOS4, but while all Apple's apps (like Calculator and iPod) are compatible, all third party apps will need to be upgraded to properly multitask. So, you'll need to head to the App Store app on your phone, then visit the Updates section to download a update for each app. This can take a while, but don't forget, the App Store app also multitasks (as it always has), so double tap the Home button to get back to it to save time after selecting each update. There's also a handy "Update all" button to tap.
Use the compass with maps.
Tap the Location button in Maps twice to use the built-in compass to orient the map based on the direction you are facing.
Lock the screen orientation
if you're lying in bed reading an iBook or a page in Safari it can be annoying when the iPhone's orientation keeps flicking between portrait and landscape. You can solve this by activating the new Portrait Lock. Double tap the Home button to reveal all the running apps, then swipe your finger to the right to reveal iPod controls and the Portrait Lock.
Make and rename folders
Organising your apps is so much easier on iPhones running iOS4. Just tap and drag apps on top of each other and you create intelligently named folders. You can rename the folder names Apple chooses though - just tap and hold on an app to make them wiggle, then tap on the folder name and start typing.
How to Tweet with Siri
If you are a fan of Twitter, you may be disappointed that you can’t send a tweet with Siri right out of the box. However, if you add the Twitter text contact, you can send a text update to Twitter using Siri.
Location, Location, Location
The biggest battery murder culprit is Apple's new push on location awareness. Reminders, Find My Friends, and the underlying software that allows Foursquare to do creepy/neat things like remind you to stop at a nearby cafe. No matter how you're using it, your new iPhone is using its GPS and cellular powers to find itself in the world way more often than ever before. That's a kick in the nads for battery. Luckily, you can turn a lot of it off.

Save images from the web.
In Safari, touch and hold an image to save it to your Camera Roll or copy it to paste into an MMS or email.

Scroll to the top fast.
In Safari, Mail, Contacts, and many other apps, tap the status bar at the top of the screen — which displays the network information, time, and battery level — to scroll quickly to the top.