When Adventure 2 was coming to its end, in a Twitch stream Minna said that the possibility exists. I seem to recall it was in connection of her mentioning that she has no intention to draw any more SSSS except if books 5 and 6 were happening, in which case she would draw dome extras. This may be a contract thing, either actually entered to or that she feels she would be duty-bound to do something.
But, all of the books so far have first been in Kickstarter. I’m not sure a book 5 would be such a huge success now that there is no updating comic to draw people to the campaign. Although I have to admit I have no idea how much of a difference it would make in reality. But is there a channel to advertise it so that it would gain traction? I don’t know.
So all in all I’m not considering it very likely they will happen, but it’s not completely outside of the realm of possibility that they might.
I also wouldn’t personally want to get book 5 before I knew 6 was / would very likely be available, because I would hate having 5 but not 6
But that’s just me.