Please provide more information. Are the drives seen in BIOS / Device Manager / Disk Manager? What properties are shown in the Disk Manager? The easiest way to tell, I would think, is to attach the drives and see what the computer thinks!
From what I see you have 4 SATA ports, therefore you should be able to use 4 HDDs. Do as suggested above: plug them in and see whats up.
I'm not sure about hardware RAID support on your motherboard, but Windows disk management under computer management should fit your needs (software stripes and mirrors). I'd stripe the two 500 GB drives for use as a OS/program drive (would have to be hardware raid), and then mirror the two 2TB drives (documents) for redundancy. You could span the two disks into a single 4TB volume, but then you should have 4TB of backup elsewhere (in an ideal world). It never hurts to back up your stuff.
Another note: are the drives you picked up truly external drives in that they come packaged in external enclosures (smart, you are saving $$$ right now)? If so, be careful to setup the drives accordingly as usually these drives are "Eco-friendly," meaning they park their heads a little too often, not a great idea for drive longevity. An example: look up WDIDLE on WD Green drives (specifiaclly EADS models).
Being SATA, they operate by their own rules. i.e. each drive is free to spin at whatever speed its controller chooses. SATA drives aren't dependent on other drives as they are each their "own channel." I've seen PATA drives (master and slave) exhibit this behavior, but never SATA drives. You should be fine.