And am I right in saying there's only 4 radiant quests for the vampires?
No.
There are many radiant quests, but there are only four magic items that help you out by boosting your vamp powers.
There are quests to beef up the Blood Chalice's effect in variable gradients.
There are the 4 magical item quests.
There are quests to visit a city and turn one of its residents into a Vampire to increase the Vampires sway in the region.
and the list goes on.
there is a ton of stuff to do for the Vampires even after Harkon is no more.