I don't know if I really have any great advice for you, but wanted to let you know that I went through the same kind of thing with my son right after he turned four, as did another friend of mine... so you're not alone.
My son said the same things like he had bad dreams...and then it "turned into monsters under the bed". He was so terrified that we couldn't even put him in his bed for awhile, as he would scream, cry and run out of his room. It was a long several months for us. I hope that your son's isn't as long.
But, several things we tried...were we talked about his "dreams" and also got different books to read from our school counselor. I didn't know where to turn after several months. This helped a bit. But, I also spent many nights sleeping by his bed until he fell asleep.
Do you have a dog? I know this will sound strange, but...My son was also in a toddler bed that sat low to the ground and we realized that our dog would come in every once in awhile and give him "kisses". Well, she is black and when it's dark you can't really see her. We think this might have been his "monster that was eating him." I know sounds funny now, but wasn't when we were going through it. :)
Then, we decided he really had outgrown his toddler bed (he didn't want to give up the race car bed) and bought him a bigger one. Surprisingly this helped him almost immediately. It's the kind that is kind of like the bunk beds and shaped like a "T", so it was almost like he was in his own little "tent". So, I don't know if it was the excitement we made of the new bed, timing or the fact that our dog could no longer reach him to give kisses. But, it worked.
Good luck on dealing with this, I know how hard it can be on all involved.