Side by Side: New and Old iPhoneThe iPhone 4 (left) is slim and svelte, with a premium feel and a stainless-steel edge. In comparison, the iPhone 3GS looks and feels plasticky. The border of the iPhone 4 doubles as the device's cellular and wireless antennas.iPhone 4 Slims Down\nThe iPhone 4 measures 0.37 inch deep, versus the iPhone 3GS's thickness of 0.48 inch. It feels comfortable in the hand, and is even slightly narrower than the iPhone 3GS.\nNew Buttons\nAs part of the refresh, the iPhone 4 replaces the volume rocker with rounded, individual volume-up and -down buttons. The iPhone 4 also has a new ring\/silent switch design.\nHeadphone Jack and Power ButtonHere is the revamped, stainless-steel-rimmed top edge of the iPhone 4, with the headphone jack and the refined wake\/power button.Stylish, Functional TrimThe stainless-steel trim doubles as the iPhone 4's redesigned wireless antennas (hence the break in the trim visible here). On the right side you'll find the micro-SIM card slot, too.New, Higher-Resolution DisplayThe iPhone 4 has a 3.5-inch, 960-by-640-pixel IPS display with 326 pixels per inch. It packs in more resolution than competing handsets, some of which have larger screens.Sharp ScreenThough some people have debated whether the iPhone 4's display truly surpasses what the retina can perceive (as Apple claims), the improvement is quite visible. Whereas the iPhone 3GS's text--in the menus, in apps, or on Web pages--appears thick, fuzzy, and undefined, the iPhone 4's text is razor sharp, even when enlarged.FaceTime: Easy AccessOne of Apple's strengths is its integration of hardware and software. The iPhone 4 is no exception: You can initiate a video phone call directly from your contacts list by tapping the FaceTime option, which launches Apple's FaceTime videophone app. FaceTime uses the iPhone's front-facing camera to conduct videoconferencing between two iPhone 4 handsets.FoldersFolders are a welcome addition, one that will be helpful for people with tons of apps. To create a folder, you simply drag one icon on top of another; the folder automatically gets the name of the category those apps share (though you can rename the folder on the spot). You can pack a maximum of 12 apps inside a single folder (for three rows of four apps across the home screen), and a maximum of 2160 apps in all.Welcome, iBooksNew with iOS 4 is the addition of the iBooks app. On the iPhone, the iBooks app looks just as it does on the iPad, except scaled to the iPhone's smaller screen.iBooks Gets PDF SupportThe major new feature in iBooks is its native support for PDFs. You'll find tabs for both books and PDFs, and each tab shows a bookshelf or list view (your choice).iBooks Gains NotesThe iBooks app can now create notes and bookmarks, and you can see those notes, bookmarks, and highlights in the table of contents.iBooks PDF ViewYou can add PDFs via e-mail or Safari, and PDFs can sync back to iTunes and to other Apple devices such as the iPad or iPod Touch.Multitasking and Closing AppsAs part of iOS 4, the iPhone 4 gains multitasking. You quickly double-tap on the home button to pull up a pane that shows which apps are open in either a live or suspended state; there you can swipe horizontally through the apps and select which one to resume. To close an app out of the multitasking bar (shown here), you first click on the icon and hold. The icons then get a red button with a dash; touch there, and you can close the app.Worth the Upgrade? YesOur early impressions of the iPhone 4 are that it's a worthwhile refresh of the hardware, and that it will deliver a measurable upgrade over its predecessors. Owners of the original iPhone or the iPhone 3G will benefit the most from upgrading, but iPhone 3GS users will find something useful here, too.