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It depends upon the MOBO and Chipsets used. For example, the Marvell 91xx are x1 PCIe lane and the maximum is 360~380MB/s in contrast the Marvell 918x are x2 PCIe lanes and the maximum is 550~590 MB/s; both use shared bandwidth. In general native SATA3 on the Intel P67/Z68 can achieve ~590 MB/s. but is limited to 2 ports.
Further, a 'SATA 6Gb/s' on a HDD or SSD refers to the Interface and not the speed of the drive. The only 4 port 'SATA 6Gb/s' that comes to mind are a mixture of Intel {not shared} + Marvell {Shared w/various number of lane(s)} but the AMD only came in 6x6Gb/s and those are shared.
So what 4x6Gb/s MOBO are you referring to??