Anything out there that suits my needs?
Probably not. You'd have found it already.
I'd remind you that the Silverstone breadbox cases (SH05/06/07) all have two expansion slots -- they are mini-DTX.
But 4 x 3.5" HDDs and fanless don't compute. Those HDDs will drown out any 120mm fan that can easily cool them all... so why would you bother trying fanless?
In the SPCR lab, we have a system that is single-fan cooled with a single platter Samsung 500gb ecogreen encased in a Scythe or Smart HDD silencer sitting on foam -- and the HDD is still the loudest component.