why does the nest2 have 4 USB ports and the nest4 does not?
http://squirrelsresearch.com/acorn-nest-x2g/AcornNest X2G allows you to host two Acorn units in any PCIe 2.0 or greater x8 or x16 slot and provides four USB PCIe riser connections for accelerated GPUs without any special motherboard requirements.
versus
http://squirrelsresearch.com/acorn-nest-x4/AcornNest X4 allows you to host four Acorn units in a PCIe 2.0/3.0 x16 or PCIe 3.0 x8 slot without any special motherboard requirements.
It includes an onboard DMA engine to reduce CPU load, as well as advanced Acorn power utilization and performance monitoring.