16x4x isn't 20x or 2x10,when a motherboard supports crossfire at 16x4x mode it means that when you put 2 cards in it in order to crossfire them,one of them will operate at 16x speed and the other one will operate at 4x,and for gaming this is not recommended,go for either Dual 8x or Dual 16x if u want to CrossFire
http://www.tomshardware.com/forum/245454-33-crossfire-faqs
"A note about Motherboards:
Some Motherboards support Multi-GPU Technology at Dual 16x mode, some support it at Dual 8x mode and some support it at 16x 4x mode Etc, what are the differences?
Well for getting the best performance out of Multi-GPU cards,you need a Motherboard that supports Multi-GPU at dual(Or more) 16x mode like Nvidia 790i ULTRA SLI, Nvidia790i SLI, Nvidia 780i SLI, Nvidia 680i( As i said, NVIDIA 790i ULTRA SLI ,790i SLI ,780i SLI and 680i SLI support 3-Way SLI too(Nvidia 680i LT SLI chipset has 2 PCI-E 16x slot which supports 2-way SLI only.),Nvidia 590i SLI, Nvidia nForce4 SLI 16x, ATI CrossFire Xpress 3200,Intel X38,X48 and X58.
Dual 8x is a very good configuration too,it doesnt have alot of difference with Dual 16x and its cheaper too.
Chipests like Intel P55*,Intel P45 Nvidia 750i SLI, Nvidia 650i SLI,NVIDIA 570i SLI, NVIDIA nForce4 SLI, ATI Radeon Xpress 200, Intel 975x and ONLY few Intel P35 boards(Like ASUS BLITZ FORMULA and ASUS BLITZ EXTREME) support Multi-GPU at dual 8x mode.
However for cards like radeon HD 48xx series,its better to get a Dual 16x motherboard because it performs better.
However,there are some expections too,for example EVGA has a 750i Motherboard(The model of it is
EVGA 750i SLI FTW)Which supports SLI at dual 16x mode unlike many 750i Motherboards which support SLI at dual 8x mode.
The last one is 16x 4x, this one is the weakest and doesn't perform very good compared to dual 8x or dual 16x,so i won't recommend it for Multi-GPU configuration. Intel P35 and P965 chipsets support Multi-GPU at 16x 4x mode.
Here is a good review which compares CrossFire performance between dual 16x,dual 8x and 16x 4x:
http://www.tomshardware.com/reviews/crossfire-pci-express,2095.html "