"PLX" is a bit of a misnomer, PLX Technologies is (was) a manufacturer of circuit components like PCIe switches and PCIe bridges and PCIe multiplexers. What "PLX" or "PLX chip" means to most people is a chip which splices a single physical PCIe slot (or lane, or group of lanes) into more logical PCIe slots or lanes.
Short version is that it doesn't matter how many physical slots are mounted on the motherboard or how many lanes can be allocated in the BIOS because the processor can simultaneously address a maximum of 40 PCIe 3.0 lanes worth of bandwidth under best conditions. There are zero performance gains once PCIe 3.0 devices saturate all these lanes (in fact, there are minor performance losses since the PLX processing part adds signal latency and complicates timings, the increased signal traffic adds packet envelope overheads, and the increased signal collisions/errors require more packets get resent). So you might be able to install four GPU cards in a PLX x16/x16/x16/x16 configuration but actual performance will be comparable to (actually worse than) a non-PLX x16/x8/x8/x8 configuration. (And, besides, even mighty 980Ti and TitanX GPU cards at full load can hardly saturate >8 PCIe 3.0 lanes in practice, especially if they're directly interlinked off the PCIe bus with a CF/SLI bridge.) Just gotta wait for future Intel processors with more integrated PCIe 3.0 lane controllers.
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