Jason,

Having a mini-SAS connection for 4 HDDs is complete overkill.

No hard drive exists which can saturate a single 6 Gpbs channel, this is something which only a handfull of SSDs can do -- and only under very selective conditions (sequential reads). The highest transfer rate that I have seen is less than 200MBs (1.8 Gpbs), so 4 HDDs could be supported (almost) by a one of the 4 channels in the mini-SAS connection.

Further, remember that SAS expanders work on the basis on taking a 4-channel connection from the main RAID adapter and allowing multiple devices to to share that connection, so your are ultimately going to run into a limit of the speed of 4 channels (4 x 6Gpbs = 24 Gbps = 3 MBs). So, using expanders will not explicitly increase your overall performance.

If you want to still pursue a 4-channel connection to 4 HDDs approach, you should think in terms of increasing the number of expanders (or using multiple controllers -- although that has it's own issue).

Personally, for a near fully populated chassis (48 HDDs) I perfer to see 4 expanders -- each connected to 3 backplane channels.

Sean

P.S. You might want to have a look at SuperMicro they have a couple of some 4U chassis (vs. 9U in the Chenbro chassis) which support 36 or 45 3.5" HDDs! So, you could have a lot of rack space.