Most likely pickup now is from Amazon, which has the rights to the first three seasons. It's unlikely Netflix would pick it up if they didn't also get seasons 1-3 as part of the deal to make a season 4.
Bryan Fuller is not signed for a 4th season, he has a show on Starz about to start up. So he's not part of the picture, or, doesn't have to be.
Also, the production company does not have the rights to make the "Silence of the Lambs" storyline that season 4 would encompass. They only have the rights to the characters through "Red Dragon", which is this season. So any pickup would probably have to involve paying for those rights as well. Presumably, however, Thomas Harris won't be able to ask as much for SotL now that a big-name network isn't involved. If he was holding out hoping that NBC would bring the money truck to his door, that plan is shot.