4 way CF and SLI isn't nearly as good of a value of 2 way CF, also only high end cards support 4 way CF and SLI. For nVidia i believe nothing below the 570 and 580 have 2 SLI connectors and for AMD nothing below a 6950 and 6970 have 2 CF connectors so 4 way CF/SLI is really restricted to the top end cards.
Its also really expensive to get a board that can do 4 way CF/SLI well since most chipsets don't support enough PCI-e lanes natively. The AMD 890FX chipset provides 38 PCI-e lanes which is enough for 8x/8x/8x/8x quad CF/SLI, but the 890GX only provides 22 PCI-e lanes which is only enough for 8x/8x/4x at best. The Sandy bridge CPUs each provide 16 lanes from the CPU, the chipset provides 8 more for non-graphics slots, so without using an NF200 chip the best you can do with a Sandy Bridge CPU is 8x/8x CF/SLI. That is why most boards are only dual SLI/CF and not quad CF/SLI capable.
im just saying weird to offer both types on 1 board what use would you ever use that in.
Ah. I misunderstood. Well in that case it's because the MB has only 2 PCI-E x16 slots so only 2 cards can be installed. What the 4-way CF/SLI refers to is a situation when you use two dual-GPU cards totalling 4 GPUs.
Why? Enabling CF is a merely a case of setting a flag in the BIOS, SLI is a small licensing fee and another flag in the BIOS to enable it. It takes no additional hardware to enable both SLI and CF if your board already has the PCI-e lanes to support it. You can only run either SLI or CF at any given time though, its not meant to support both simultaneously.
The reason for that board only supporting dual CF/SLI and not quad is how they arranged the lanes, the top two are always 16x and the bottom two are always 4x. Some 890FX boards auto switch the top two to 8x when the bottom two are also in use to provide 8x/8x/8x/8x or 16x/16x/8x for triple CF/SLI.
Its basically letting you choose whether you want ati or Nvidia cards,
instead of just offering one compliance, they offer you both and so you are not tied to any particular gfx manufacturer (which you may not like)
Hmm. Maybe I misunderstood again. I just remember rather devious marketing when two-way CF/SLI was marketed as 4-way with a disclaimer that 2 dual-GPU cards were necessary for 4-way.
It's not waste of space in the sense that it gives the buyer the possibility to use multiple graphics cards from either AMD (CF) or Nvidia (SLI). If an MB doesn't support SLI then you can only use 2 or more AMD cards in a multi-card configurations. As far as I know every MB that supports SLI also supports CF. But not all boards that support CF also support SLI. It has something to do with Nvidia asking for royalties from MB manufacturers if they want to enable SLI. AMD does not ask royalties for CF.