The S4's eight-core design is actually based on AMD, which basically means that there are 4 powerful A15 cores for powerful processing speeds. There are also four lower power cores, the Cortex-A7, for the S4 to use when it doesn't need the A15 cores, thus expanding battery life.
The S4 will include an impressive 5-inch display with a high-resolution of 1920x1080, with a ppi of around 440, which is actually much better than Apple's Retina Display.