re: OK done with the PS4 for now

First Party - advantages are that the owning publisher (Sony, Nintendo, Microsoft) are responsible for the costs and logistics associated with distribution, marketing, and manufacturing of the product. Additionally, the publisher has the resources to provide equipment and technology (like motion-capture) that the studio would otherwise not be able to afford. Lastly, First Party developers can focus optimization on one particular platform and receive guidance/assistance as needed.

Naughty Dog is a great example of the above. They went from making Crash and Jax/Daxter games to Uncharted and TLOU series thanks almost entirely to the logistical and financial support of Sony. Same thing with Guerilla Games; no way they would have had the resources to go from making Killzone games to Horizon Zero Dawn with Sony's support.

Are you wondering why these companies tie themselves to just one publisher? Hell, risk is part of any business. There's been thousands of "independent" game companies that have come and gone over the years. Nothing's guaranteed. I'm sure each studio and each deal have unique situations that everyone involved must weigh. In Naughty Dog's case, they clearly made the right choice to join Sony.