Splendid is the German distributor, they just bought the German rights, like they did with the previous three. Millennium is still producing.
All past beefs aside, if it makes sense for them they will get Seagal. Sly has gone on record saying he has reached out to Seagal in the past but he declined.
If, and that's a big if, they still produce a fourth one and if they still reach out to Seagal, at this point he will probably say yes. Let's be honest, it would be the absolute last chance for him to get a little boost to his career before it's all over. Judging by his recent tweets he has changed his mind and now seems to want to do it.
Seagal is one of the last well known action stars yet to appear in the franchise, along with Jackie Chan. It would make total sense for him to do it.