I hope I'm not being repetitive by mentioning Tony Burrough's sets again, but they really are the star of this story for me. Even more rooms on the Urbankan ship are seen in this episode, including the Flora Room, the Recreational Room, and the very spartan quarters that Bigon once inhabited, but a room to which The Doctor and co. are assigned.
These sets are just as impressive as the ones seen in Episode One. The walls are exquisitely detailed, and have clearly been influenced by the designs seen in the corridors of the Death Star in Star Wars. That's not an insult towards Burrough, either, because to replicate the look of Star Wars on the budget of Doctor Who is a definite sign of skill. Another strong quality of the sets is the fact that most of them are two levels high, often punctuated by a staircase and a balcony. This not only allows for a greater variety of movement for the characters, but it also enhances the scale of each room. Even Monarch's throne room, although one of the only ones that exist on one level, seems grandiose because of the monolithic wall in behind the three chairs that Monarch and his two minions sit in for the bulk of the story.
Another impressive, yet simple, visual aspect of this story is the realization of the monopticans, those mobile CCTV orbs that follow The Doctor and gang around, much to the annoyance of Adric. Sometimes they're CSO'd onto the action, sometimes they're simply propped up on an invisible pole in the background, and, in the case of the one in The Doctor's quarters, it's hanging on unseen strings so the Doctor can slap his hat on it to stop it from spying on his actions. Simple, yet effective, the success of the monopticans are symbolic of the subtle visual brilliance that Four To Doomsday exudes in waves.