@evleaks who has been very reliable with leaks in the past is reporting that Samsung's Galaxy S4 will feature a Super AMOLED Full HD screen, 2GB of RAM, a 13-megapixel autofocus camera (4128x3096 pixels) with LED flash, Android 4.2 JellyBean and come in 16/32/64GB versions.
They also posted the above images that reportedly show how the device will look. The first clearly showing that the S4 will be bigger than the previous previous Galaxy S devices. It also appears that the S4 will look more like an S2, by body shape at least.
While we can't be 100% of these days, I'm very confident that they'll be pretty accurate considering @evleaks' record of accuracy with previous smartphones.