Samsung has high hopes for the Galaxy S4 (they actually hope to sell over 100 million units - or double the GS3 sales). Today the company reported that they will soon reach 10 million units sold - less than a month after launching the phone (update: on May 23 Samsung announced that indeed they reached the 10 million sales mark).
The GS3 took 50 days to reach 10 million in sales, so the GS4 is selling at a much faster rate (almost double the rate actually). Google just announced that they will offer a GS4 model with stock Android 4.2 on their Google Play on June 26 for $649, this won't hurt sales rate I guess.
The GS4 features a 4.99" Full-HD (1920x1280, 441 PPI) Super AMOLED display, an Octacore 1.6Ghz Exynos CPU (some models use a Quadcore 1.9Ghaz Qualcomm CPU), 2GB of memory, 13 mp camera and a 2,600mAh battery - all this while being a smaller and lighter than the GS3. The GS4 includes a lot of new software features and special UI controls, include the Adapt Display which allows extensive display calibration adjustments.