Yeah but 1 dude doing 4D stuff doesn't mean all other demons are doing 4D stuff if they're using the same energy. Agrosax did some time hax, mundus created a universe, but it was never implied he created a timeline. Other demons affecting time doesn't mean Mundus also created a timeline.
I already answered this, all 3 realms( Human, Demon and Mirror Realm are separate space-times) , Mundus busting out a separate realm will have to be a Low2C feat.....the realm does not share any connection with others....its equivalent to a Big Bang which initiates space-time and matter only in this case Mundus uses Demonic Energy and no explosion is required.
I don't know where you are going with this "not same energy" when he is literally the King of the Demon World....guy who when he was about to die made the Underworld collapse , the guy who was sealed in a Marble statue
INSIDE the Demon realm ..... the guy comes out of the Statue and first thing he does is bust out a realm....
And even If I steelman you and say that he did not use the "same energy" let me remind you that Mundus slew the Old Demon King Pluto, ate the Qlipoth Fruit which is superior to the Tree which can merge the realms, Mundus didn't have control of Demon World Energy at that time and yet he won....which means his own power scales to the "energy" which will give it the same properties....
Later down the line Argosax who was an Old God only managed to stalemate Mundus , Argosax didn't have control of Demon Energy at that time but Mundus did.
Abigail is a direct rival to both of them and he didn't have control of Demon Energy at that time but Mundus did.
Same with Sparda when he defeated Mundus.
Might wanna take a guess who is Demon King of the Underworld in DMC1??
Even if they don't use "Demon World Energy" and use their own it is still scales to it....they can outright match and overpower it as shown in multiple instances....which is impossible if it isn't 4D.