By Melissa J. Perenson, PC World

The Apple iPhone 4 is everything that a new piece of technology should be: It’s innovative, attractive, and ahead of its competition. In comparison, previous iPhone upgrades seem inconsequential–that’s how much iPhone 4 brings to the table.

The phone will ship on June 24, priced at $199 for the 16GB model and $299 for the 32GB model (in white or black).

Premium Look

I spent some hands-on time with the new handset at the Apple event. I’ll start with the visuals: It’s stylish. Whereas the iPhone 3GS looks and feels plasticky, the iPhone 4 is svelte and has a premium feel. Surprisingly, it achieves that impression while retaining the same general design, although the edges appear a bit more squared than before.

It’s noticeably slimmer than the iPhone 3GS, measuring 0.37 inch deep versus the iPhone 3GS’s thickness of 0.48 inch (that translates to 2 percent less). The iPhone 4 is also slightly narrower, 2.31 inches to 2.44 inches. The weight stays the same at 4.8 ounces, but the tweaks to the dimensions make the current iPhone 3GS seem downright kludgy in comparison. (See all iPhone 4 specs.)

However, it’s the aesthetic design touches that make the iPhone 4 stand out. The overall design screams elegance–from the rounded, individual volume up and down buttons that replace the plastic volume rocker on the iPhone 3GS to the ring/silent switch and the power/sleep button up top. The face and back are made of glass that is specially treated to withstand scratches and oily fingers, according to Apple. The side edging is aluminum, and doubles as the device’s three cellular and wireless antennas.

Sharp Display

Of course, the iPhone 4 isn’t just about cosmetic enhancements, pleasing as they are. What makes this phone such a technological improvement is what’s inside the handset.

Like its predecessor, the iPhone 4 has a 3.5-inch display. But the new phone’s display doubles the resolution to a 960-by-640-pixel IPS display. At 326 pixels per inch, this is the highest resolution available on a phone to date.

That display truly makes a difference. 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 (as I tried doing when viewing a PDF).

The new “Retina display”–so named because it surpasses the number of pixels the human retina can process–also greatly improves the sharpness, clarity, and visible detail of images.

In both cases, I’d liken the magnitude of difference to that between a standard-definition 80p DVD and a high-definition 1080p Blu-ray Disc: When you view both on an HDTV, the differences are striking. And once you see them, you can’t go back.

The real value of the new display will become evident for people who spend time reading on the iPhone 4. I expect the display will make reading a more pleasurable experience (although, clearly, limits will remain given the inherently modest screen size–modest, at least, as compared with handsets such as the Sprint Evo 4G, which has a 4.3-inch screen, and the much larger 9.7-inch iPad screen).

iBooks Goes Mobile

The high-res display, coupled with the addition of iBooks on the iPhone 4 (and with iOS 4 upgrades), makes the iPhone a more relevant e-reader. iBooks retains its structure, appearance, and function from what we’ve already seen on the Apple iPad; and with this OS’s ability to sync the iPad, desktop, and iPhone, readers gain the flexibility to move seamlessly among devices. This capability is available for Amazon’s and Barnes and Noble’s respective e-readers, as well, but not for other competitors.

iBooks also gains a few new features previously unavailable on the iPad. You can now create notes and bookmarks, and see those notes, bookmarks, and highlights in the table of contents. I suspect that the notes remain trapped in line–for example, there’s no way to create cheat sheets, summaries, or other such personalized shortcuts that you could then utilize on your computer or elsewhere–but these new functions are a step in the right direction.

The major new feature in iBooks is its native support for PDFs. You’ll find tabs for both books and PDFs. Each one gets a bookshelf or list view (your choice). You 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.

iPhone 4: A Computer in Miniature

The iPhone 4 uses Apple’s A4 CPU, the same processor powering the Apple iPad. And it runs the newly renamed iOS 4 operating system (which the iPad will also use, starting in the fall).

As part of iOS 4, the iPhone 4 gains a bevy of capabilities. One of them–multitasking–feels long overdue, but as with Apple’s long-awaited cut-and-paste feature, the company delivers on the promise of making multitasking work smoothly.

Quickly double-tap on the home button to pull up a pane that shows which apps are open. From there, you can swipe horizontally through the apps that the iPhone 4 has retained in either a running or suspended state.

When you find the app you want, you just click on the icon. The app will then resume its activity, and, if written to take advantage of this new feature, it will pick up precisely where you left off. At the very least, reaccessing the app will be faster.

Comparative Use Tests

Let’s take the example of the side-by-side tests I did with an iPhone 3GS (running iPhone OS 3.1) and the iPhone 4. I navigated between the Safari Web browser and the Photos application and back again to Safari, and then back again to Photos.

iPhone 4: On the iPhone 4 using iOS 4, the phone jumped quickly and smoothly between the apps, with virtually no pause or hesitation. I left a fully drawn Web page in Safari to go to Photos, navigated to a folder within Photos, and then to a picture in the middle of that folder. When I popped back to Safari, I resumed at the fully drawn Web page, and when I jumped back to Photos, I was looking at the same photo I’d left moments earlier.

iPhone 3GS: That same exercise on the iPhone 3GS required the Web page to draw the first time. To change apps, I had to press the home button to exit Safari. I then went into the Photos app and found my image in its album. To go back to Safari, I pushed the home button to return to the home screen and then clicked on Safari. (On one pass, the page loaded immediately; on another, it did not). I then pressed the home button to return to the home screen, selected Photos again–and found myself back at the top-level list of Photo Albums, as opposed to drilling down to a specific image within a specific folder.

To close an app out of the multitasking bar, you click on the icon and hold. The icons then get a red button with a dash; touch there, and you can close the app.

Equally as elegant as multitasking is Apple’s implementation of Folders, an increasingly necessary addition. To add icons into a folder, you simply drag one icon on top of the other to create the folder; the folder automatically gets the name of the category those apps share. Or, if you prefer, you can rename the folder on the spot. You can pack a maximum of 12 apps within a single folder (that gives you three rows of four apps across the home screen). And, thanks to the addition of Folders, you can now add up to a maximum of 2160 apps.

Dramatic Camera Boost

The iPhone 4 brings much-desired camera and video recording advances, as well. The primary camera on the back bumps up from 3 megapixels to 5 megapixels, while retaining the same pixel size (which can further improve image quality). The camera also gains an LED flash, a backlit sensor, and an integrated 5X zoom. The camera now lets you shoot in high-def, at 720p, 30 frames per second; in addition, video gains the tap-to-focus feature already available on the camera.

I did not test these features–the lighting at the demo room was a bit funky, and I would have only been able to view the results on the demo device. However, the examples that Apple showcased during its keynote were compelling evidence that these upgrades are indeed worthy ones. These will be among the first features I’ll try when I get my hands on a device for our full review.

I didn’t fully test the front-facing camera, another addition to the iPhone 4, either. This camera is integral to Apple’s FaceTime videophone app, which works only for communicating between two iPhone 4 handsets.

iPhone: Upgrade?

From my early look at the iPhone 4, this handset appears to be a must-have for anyone with an original iPhone or iPhone 3G (the former won’t get the iOS 4 upgrade at all, while the 3G won’t support some features). And people who have an iPhone 3GS will find this a worthy upgrade, too.

Unlike the previous jump, from the iPhone 3G to the 3GS–which focused on slight performance improvements–the iPhone 4 bolsters the hardware’s digital imaging capabilities and its display, making it a comprehensive and measurable upgrade over its predecessor.