4-way SLi doesn't exist. 3-way is the max at the moment. Only the , 8800GTX, 8800 ULTRA, 9800GTX, GTX 260 and GTX 280 support 3 way SLi, all other Nvidia cards support 2-way only.
The video cards must be identical to work in SLi mode.
whats the difference between sli and crossfire?
Crossfire is AMD/ATi's multi-GPU technology. SLi was 3DFX's, it's now Nvidia's.
I'd say CF is superior in a few ways. All cards HD 3800 series upwards support CF with up to four cards at a time. Non identical cards can work together in CF. (Ex: HD 3850 + HD 3870 etc).
CF is supported on many more chipsets than Nvidia's SLi which requires an Nvidia chipset. You can CF ATi cards on an Intel P35/P45/X38 or X48 board. And Intel chipsets are a welcome alternative to the Nvidia's crap especially as you're OC'ing.