I'm looking to use an external 4-bay enclosure to hold 4 SATA HDDs.
Enclosure can use either eSATA or SAS connectors, but I need either PCI Express x1 card or PCI only (no other slots will be available on mobo).
Trying to avoid SIL based controller since my Hack will have 8GB RAM & we all know about the KPs with these chipsets with more than 8GB RAM in Hacks . . .

You should get a 4 or better 5 bay enclosure with eSata port multiplier support, then grab a Sonnet Tempo Sata X4P eSata PCI card. This one delivers 4 external eSata ports with port multiplier mode, which means it can support up to 20 drives via those 4 eSata connectors. I use the PCI-express version (E4P, but 4xPCIe), which works wonderfully under Kalyway 10.5.2 and Sonnets original drivers.