Just last week, my almost-4 year old pointed at the end of the hall and said he was scared. We finally got him to say he was scared of the dinosaurs in the wall. My DH turned on the hall light, said he didn't see any dinosaurs, and asked DS to show him where they were. DS pointed to a spot about head high on the wall and said they were behind the wall. (DH and I exchanged one of those, where did that come from looks.)
First, DH turned on the two bedroom lights, then he showed DS that there was another wall behind it in one bedroom, and the closet was behind the dinosaur wall in the other bedroom. He was trying logic to say that no dinosaurs could fit in the small wall. DS was still scared. DH thought for a second and proclaimed that DS needed a flashlight to fight the dinosaurs. Off they went to grab the flashlight. DS turned it on and shined it on the dinosaur spot while DH made various sound effects. For the coup de grace, DS gently hit the wall with the flashlight, and DH cheered at DS's victory over the dreaded dinosaurs. Afterwards it was clear DS had really enjoyed being empowered to take care of the dinos on his own. He's done the flashlight thing once more this week so far.
Other remedies that might work are monster/dinosaur spray (can be plain water in a little spray bottle), or having your DS change into his superhero persona (you know, when he's invincible), or a magic wand (my DS's magic wand of choice is any one of my 6 colorful crochet hooks).... Or make up a special song he can sing to scare away the dinosaurs?
I'm assuming you've already tried using a nightlight, and looking under the bed/in the closet for dinos is part of your bedtime routine?