I've gone during Halloween. It is only scary in the mazes that you must queue for and enter of your own choosing. It is not scary just walking around the park like Universal or Knott's Scary Farm. I like it because it is a mix of cute and scary.
So I'd say yes, you can bring your 4 year-old, but maybe not inside the mazes. We were behind a family of 4 with young kids in one of the mazes and the entire family was so scared they were walking around hugging in a circle.
During the day, there will be games to play in the Grizzly Gulch area. Easy Autumn Fair type things and the prize will be... what else? Stickers! The mazes are open during the day. Again, they are completely avoidable if you want. There will be some Halloween themed food, especially dessert, in the parks. The bakery on Main Street willl have Halloween stuff. There's a cafe on Main Street that has a Halloween High Tea. There are popcorn buckets with a plastic trick or treat carrier to hold the popcorn. The Enchanted Garden at HKDL Hotel has a Halloween themed dinner buffet. I am pretty sure Mickey and Minnie are dressed up for the season for photos.
At night, on Main Street there are people in some kind of Victorian animal type costumes walking around, posing for photos. About every 30 minutes or so they do a little choreographed dance. These costumed characters (I want to say they were werwolves? Cats? It was hard to tell.) do NOT go up to random guests and try to scare them. They do their dance and then they are available for photos and will pose in a "scary"way, but only if you seek them out.
There is a short parade with Jack Skellington that is cute. I don't remember fire works. There is a VERY minimal trick or treating. If you have been to the party in the US, it is nothing like that. There are like 4 stations with candy or stickers. There are places to take photos set up as well throughout the park. There is no extra ticket for the party. It is all one cost.
I hope that helps.
