MacConnection offers the Seagate 4TB Backup Plus USB 3.0 External Hard Drive, model no. STCA4000100, for $139 with free shipping ($0.04/GB). That's the second-best price we've seen for a 4TB USB 3.0 external hard drive; we saw this one for $10 less two days ago, since expired. (Excluding the price below, it's also a current price low by $10, although most stores charge $179 or more.)

A close price: Buy.com via eBay offers it for $139.99 with free shipping.