Samsung says that orders for the Galaxy S4 so far has been "explosive". In fact the company didn't anticipate such strong orders and cannot meet demand (they have a shortage of memory chips, apparently). Samsung have high hopes for the S4, and they hope to sell over 100 million units - that's double compared to the S3 (which sold 50 million units so far).
In fact all the phones in the Galaxy S series together just recently crossed the 100 million units mark. In January 2013 Samsung announced that the GS2 sold 40 million units, the GS3 sold 30 million and GS1 sold almost 30 million.
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.