Amazon offers the Seagate 4TB Backup Plus USB 3.0 External Hard Drive, model no. STCA4000100, for $129.99 with free shipping ($0.03/GB). That's $10 under our mention from three days ago and the lowest total price we've seen for a 4TB USB 3.0 external hard drive. (It's a current price low by $19.) Sales tax is added where applicable. It's compatible with Windows and Mac and features two backup scheduling options, social network sharing, and more.

Update: Amazon has since sold out; meanwhile, MicroCenter has it for $129.99 with free in-store pickup.

Update 2: It's back in stock at Amazon.