They have not yet decided, yet written/filmed, if there will be a season 4. However, Tasie Lawrence (actress of Mara) tweeted that if there is enough demands for a season 4, that they will make one, but Tasie will not be on House of Anubis so this means that 更多 than likely Mara will not be on House of Anubis.
As for if Nina will be in season 4, it's not likely. Natalia Ramos (actress of Nina) 说 that she no longer wants to play the part of Nina (at least this is what I've heard, IDK if that's true). If it is true, then Nina will no longer be on the show. Since Nina was not on season 3, then she may not be in season 4.
The good news is that there WILL be a movie! Alex Soyer (Actor of Alfie) 评论 on a tweet that asked if there will be a movie saying, "IDK when it will air". So unless Alex is lying, there will be a movie but the 日期 is unknown. The odds of Natalia (Nina) being in the movie is not likely and the movie will be Tasie's last appearance on the show, and 更多 than likely Mara's?