The Galaxy S4 sports a 5-inch full HD screen with a resolution of 1080x1920 pixels, and pixel density of 441ppi. The S III on the other hand, has a 4.8-inch screen pixel with a resolution of 720x1280 pixels, and pixel density of 306ppi.
Talking about power under the hood, the Galaxy S4 will be powered by 1.6GHz Exynos octa-core processor in some regions, and a 1.9GHz Qualcomm Snapdragon quad-core processor in others. It has 2GB of RAM and a PowerVR SGX544MP3 triple-core GPU. The Galaxy S III is powered by a 1.4GHz Samsung Exynos 4 processor, with 1GB of RAM and a Mali-400MP GPU.
Both the Galaxy S4 and S III come in 16GB, 32GB and 64GB variants but also feature a microSD card slot for expanding the memory.
The camera of the Galaxy S4 is also superior and sports a 13-megapixels sensor, compared to the 8-megapixels sensor, on the Galaxy S III. The S4 also offers more camera features including dual-shot, employing the use of both front and back camera, an eraser shot mode to remove people from photos, and a Cinemagram like feature called Cinema Photo, among several other tricks.
The Galaxy S4 features eye control features like smart pause and smart scroll. The phone also has a sensor that lets users move their hands to the left or right to scroll between different websites they have opened or through songs or photos in an album without having to touch the phone and even preview them without opening the actual files. We're not sure if the Galaxy S III would get all the features but past leaks show some evidence that it might get some if not all from the new features.
The Galaxy S4 also features temperature and humidity sensors, which were not present on the S III.
All in all, the upgrade to the Galaxy S4 is an incremental one in design terms, but Samsung has upped the internal specs to match other modern features.