Yesterday, Sanzaru Games reported on Facebook that they are not currently working on a new Sly Cooper game, nor do they have any plans to. They are also not currently working on anything with Sony Computer Entertainment.
This doesn't mean that there isn't a Sly 5 coming. Sanzaru cannot speak for Sony, who owns the IP. But if there is a Sly 5 in development right now, Sanzaru isn't a part of it.