Will there be a sly 4?
yes, well definitely most likely. Sucker Punch has pretty much told us that they are going back to the Sly Cooper series. Theres no trailers, titles or anything just yet. but they have said a few things on the matter. like graphics being 'majorly overhauled', 'gameplay more focused', stuff like that.
and if u have played Sucker Punch's other more recent game, "InFamous", u would see that there are numerous 'Easter eggs'/ hints of Sly Cooper coming out.
So yea there will be a Sly 4 I believe.
If its coming out, I think your best bet will be sometime in June of this year, when the next E3 is going on.
If it doesnt come out then, there should at kleast be a trailer or something out about it by that time.