According to producer Jonas Rivera, who was an intern at Pixar back during the original Toy Story…
I asked the director and producers of Toy Story 4, that exact question when I had an opportunity to speak with them last month.
Unsurprisingly, nobody was quite ready to call Toy Story 4, the definite ending of the series, though they did all agree that if it turns out that way, they think fans will be happy with where the story ends.