KH4 Has to do with Sora's Darkness
I think or honestly hope that KH4 central theme revolves around Sora's and his own darkness. Now at this stage Sora already knows about light and darkness and if he were to struggle somewhat with it now, then he would have more shame to himself because how could he—someone who is part of the light wielders — struggle with this, he's supposed to be light. And yes he does accept Riku but to him Riku is different.
And let's face it Sora would accept whatever Riku has or does with darkness or not. It doesn't matter if he changed, it doesn't matter if he was with the organization, it didn't matter if he uses darkness because he's Riku.
Also when looking at Darkside, it seems to be more monstrous and the heartless are more like that but humanoid as well. Also note that darkside is because of Sora's darkness as said in re:coded and all dark sides are because of him and what if he is like Vanitas and the heartless comes because of him now since it's because of keyblade wielders that darkness appeared in quadratum. This means the darkness could be because of Sora or at least a big factor of it is (since there are other keyblade wielders like Strelitzia)
What then?
