i think the performance of 4 4890's will not be any where near to 2x 4890x2's....
unfortunatly once you get to tri quad you go to 8x pci-e and with 4 slots being taken some mobos go 4x....
(obviously motherboards differ)
you get to a point where there is not enough performance to warrant the extra card+cost
bench 4890's in crossfire Vs tri-crossfire Vs quad crossfire on same mobo, see the difference