Karthus is in a really interesting niche. He has strong area of control, and incredibly dueling potential. His build paths are numerous, and his play pattern is healthy. If you've ever seen one Karthus 1v5, then you've seen them all 1v5.
I personally don't think Karthus needs any help. The ability to smash down 80% of AP multiple times within a 5 second interval for a low mana cost is pretty good, and his Defile had a wonderful area of effect. The down side is that it can be tough to hit.
Granted, he struggles against mobile assassins and fighters, but that's normal with immobile mages. The idea is to keep them week when they can't hit you, but if they get a hold of you, you can kiss yourself goodbye.
As for the other three champions.
Xerath was reworked a long time ago, and it's because he wasn't satisfying to play as. Players had to sacrifice so much in order to be that long range artillery mage he was meant to be, so it needed to be done.
Malzahar and Zyra need work done because they have similar problems. Zyra doesn't use plants in a satisfying and meaningful way, and her combos leave her pretty straightforward, without much way to branch off into new things. Malzahar simply has little counter play, while also having near useless minions (unless AD Malzahar is your thing), which leaves him with a very awkward kit.