You have two PCIE 3.0 16x slots. When both are populated, they will be running at 8x each.
Those other two slots are PCIE 8x slots, and take lanes from the PCIE 16x slots. If you populate them all, you will be running at x4, x4, x4, and x4.
The i7-4790k only has 16 PCIE 3.0 lanes available. The Z97 chipset has an additional 8 PCIE 2.0 lanes, which is what those PCIE 1x slots use. For SLI GPUs, you're limited to two running at 8x and 8x max, unless you get into the whole PLX chip nonsense which I would never recommend. Crossfire allows GPUs to run in slots as slow as x4 though.