If you:
A. Can get a FP+ for TSM, and this is a must do.
B. Have an extra ticket that will go to waste anyway.
then, yes, I would go.
You will only have 4 hours; though, since the park closes early at 7 that night.
Some of the things I like at HS are the Car Stunt show, the Indiana Jones Stunt show, the Animation Academy, the Star Wars ride and the Jedi Training Academy.
Indiana Jones isn't playing, but the car show will be on at 4, so you could hit that. The Animation Academy is fun. They walk you through drawing a character. You don't need any real art skills to pull it off, just good listening and following direction skills. A lot of people don't know about this class either, so you can usually get in without too much of an issue even during a crowded afternoon. I think the Star Wars ride is fun, and we usually try to do it more than once, as it is a different story every time you ride. We also usually try to make time to watch the little kids fight Vader at the Jedi Academy. It is so cute and entertaining. We normally use this time for a snack or a lunch break at the QS place next door. Its the perfect viewing spot to sit and watch them.
If none of that appeals to you, there is also a Little Mermaid show, a Beauty and the Beast show, and a Frozen Sing-a-long as well. Im not a big fan of first two. I like the Little Mermaid show, but it is more because of nostalgia, then anything, and I really dont care very much for the Beauty and the Beast show. I find it underwhelming and feel I have seen better high school plays; however, the rest of my family and friends just love it.
I would suggest that if you do the sit down meal, you skip Fantasmic and make the reservation for as close to 7:00 as possible. This was the one restaurant that took us forever. We spent about 4 hours here one lunch which I don't think is normal, but still I'd think 1.5 hrs to 2 hrs would not be unreasonable. Since, you only have 4 hours to begin with, I wouldn't want to chance the meal taking up the majority of my time.
Another option, would be to save HS for a different trip when you could devote a full day to it, and just use the 4th day ticket to go to one of the parks you are already planning on visiting. You can't even see half of what there is at MK or Epcot in one day alone, so using the ticket to add a second day to either of those parks makes sense. Epcot is open to 9, and MK to 10, so you would have quite a bit more time to spend at either of those parks. For instance, you could watch the parades and fireworks at MK that first night, and then during the parade/firework time on your second night, you could skip those and take advantage of shorter lines while everyone else is watching.