Hi Jennifer!
Welcome to planDisney! I was lucky enough to go to 
Jollywood Nights last year…and I had a BLAST! I’m so excited that you’ll be checking it all out this year!
I’m happy to report that regular park guests and those with Jollywood Nights tickets will all get a chance to see 
Fantasmic on party nights (pending capacity). But I love the idea of booking a 
Fantasmic Dining Package to ensure that you get priority seating since you’ll be able to get into the park even earlier with your 
park hopper!
I actually just booked a Fantasmic Dining Package for my family’s upcoming trip, and I didn’t notice any availability for dinner on party nights. I’m guessing that’s because Fantasmic is usually shown earlier in the night when Hollywood Studios hosts Jollywood Nights. And when booking that package, they want to ensure you’ll have enough time to enjoy your meal without needing to rush over to the show. 
With that being said, lunch reservations for Fantasmic Dining Packages usually go up until about 3:55 PM. So, you may be able to snag a reservation right around 4 PM, like you’re hoping! When peeking at availability during party nights in mid-November, I’m seeing some availability between 3:30 and 3:45 PM. 
In addition to snagging priority seating for Fantasmic, getting a good meal in before party time will be a game-changer. That way, when the start of the party rolls around, you can have fun with all of the party offerings instead of taking time to sit down and eat. Remember to take advantage of all the fun 
PhotoPass spots around the park during the party because photo downloads from the night are included in your party ticket!
I hope that helps…and that you have such a magical night! Let us know if you have any other questions.
Jenn J.