The 4G looks a little like the previous 2G. The main difference (other than the shape) is an additional button on the top of the 4G shuffle. This is the new 'VoiceOver' button, something taken from the 3G Shuffle. Press and hold this button, and a voice reads out playlists that you can select. This means you can create and drag playlists from iTunes and play them on the Shuffle. I can have a fast playlist for my workout, and a slow playlist for stretching out after. Nice :o)
A big part of what helps keep my Shuffles alive longer than any other MP3 player is the build, and with the 4G, it is solid and tight, without any gaps or holes where moisture can easily penetrate. The only hole is the 2.5mm headphone jack, and that's plugged up when you insert your earphones.
My iPod Shuffle has 2GB of memory and 15 hours of battery life. It comes in grey, orange, green, blue and pink. It costs US$49.99 from Apple, and is available here on Amazon:
Update Dec 23 2010:
Ok, so it's been about 3 months since I bought my 4G Shuffle. During the second week, I managed to drop it from my dresser (about 4'/1m) and it landed on the corner where the headphones go. I dented it such that the headphones couldn't go in. It needed a small prying tool to widen the hole again. No big deal, but it goes to show you how thin the material is.
Last week, my 4G died, and I sent it back to Apple. I was concerned that it was corrosion, in which case the warranty is void. It turns out that the battery was faulty, wouldn't charge properly and was giving me about 3 hours of play. So they kindly changed my 4G Shuffle :o)