If you've already purchased films on Sony's online store, you'll be able to upgrade to the 4K version for a "discounted" price. You'll also get four titles for free if you purchase a new 4K Sony TV this summer. Additionally, Ultra is Ultraviolet compatible, so you'll be able to link to your existing library of films.
It sounds like you'll only be able to stream Sony-owned Ultraviolet content, though.
Update: Jake Winett from Sony Pictures confirmed at an event in San Francisco that you'll only be able to pull in your Sony studios Ultraviolet movies, at least for starters. He did say they'd like to add content from other studios, but it won't be there at launch. Winett also said that it was technically very easy for Sony to add the option to rent movies and that the focus on purchases is a "commercial decision." If Sony seems consumer demand for it, however, rentals may show up on the platform at a later date.
$30 for a 4K digital purchase does sound pretty steep, but it's cheaper than shelling out $400 for Samsung's 4K Blu-ray player and paying a bit more for discs. Of course, you'll only be able to get Sony's films on Sony's service. Still, if Ultra ends up being easy to use, I wouldn't be surprised if some customers ended up buying several titles. Expect to see plenty more 4K streaming services this year, in addition to Ultra and Vudu's offerings. And don't be surprised if they end up leading consumers away from 4K Blu-ray.