Two years ago, when my little guy was 4, he didn't make it into the stretch room. He yelled "too dark, too dark" at the top of his lungs and bolted for the exit.
We got him on it last year, when he was 5, and he loved it, realizing that its fun scary, not scary scary. Prior to our trip, we watched the youtube video of the ride, which was helpful. We rode it during daylight hours which seemed to help a lot. It doesn't seem as spooky walking up to it in daylight.
We also bribed him to go on it with a pin(ie. if you give it a shot and do a good job, you can pick out a haunted mansion pin). Kind of like a little badge of courage. I know it sounds awful but it worked. Only problem was, after that, the kid tried to talk me into getting him a pin to document his bravery after every ride, even freaking Pooh.