The PlayStation 4's only model at launch contains a 500 GB hard drive, but users can swap out the storage for a hard drive of any size, a Sony representative confirmed to Polygon today.
Sony is continuing a trend from the current generation: Every model of the PlayStation 3 included a user-replaceable 2.5-inch hard drive.
The Xbox 360 hard drive is replaceable, but only with a proprietary Microsoft unit, not any standard 2.5-inch drive. The same is true on Xbox One, but the upcoming console will allow users to plug in a USB external hard drive that can be used for the same functions as the internal drive, including game downloads and installations.
You can read up on more details about what's in the PS4 console and box here.