"Is it worth it?" is one of the difficult questions around the parties.
For 4 year olds, I probably would skip it in lieu of keeping on sleep schedules - ESPECIALLY since it looks like you're DVC owners so aren't expecting this to be their one and only visit. If they are BIG TIME into characters, though - and you plan on spending most of your time doing is getting autographs and pictures, then it might be.
Why I like the Halloween party for myself is the special fireworks, the AWESOME parade, and seeing some of the other guests creativity in their costumes. They used to have storytellers everywhere (Including on the boat around Tom Sawyer Island (I can't remember the name right now...)) that I really enjoyed as an extra. And the last time I went a few years back, they had a special pirate band outside Pirates. But for me, I don't need the candy from the trick-or-treating, and I'm not huge on character meet & greets. Plus I DO have an opportunity to express my personal creativity in costumes for Mardi Gras (also helps that the last time i went - NOBODY got my costume* - at all). It's also nice for me as an adult to have the later hours (hours are sometimes short in October).
But I've gotten to the point that I may do it every 4 or 5 years, but otherwise I'd rather have an early evening and an adult beverage at my resort, so I could park storm at rope drop.
*The last time I did the Halloween Party I was a "Millionaire Mourner". Who Wants to be a Millionaire - Play it! had closed in August, and this was the Halloween right after -- and I was mourning the loss of my favorite attraction. I had a Millionaire hat, with a black ribbon around it. A Milliionaire Shirt, with a black arm band, and a lanyard, with a black ribbon around the card on the end that I'd written RIP on it in white fabric paint. I often have to explain my Mardi Gras costumes, though too - so I'm used to being very obscure in my creativity.