Personally, I would go with 7DMT as an introductory "thrill ride" for a 4 year old. It's super cute, quick, smooth, and has a secure feeling to it. I thought Barnstormer was fun but bumpy. It left me feeling more jostled around than I expected from a kid coaster. I think after 7DMT might be a good time for Barnstormer, because then you could say, "This will feel EASY now!" and have two rides under his belt. After Barnstormer, I would do Splash and then, finally, ThunderMtn - which seems speedier and small bodies tend to slide around on the benches if there is only one other in the seat.
I think Slinky is an awesome ride, but it's high, and may not be the best for the maiden voyage - it might be a "work up to" ride once he's feeling confident and assured. It's so fun to watch them build confidence and excitement and of course, if he's not quite ready, there are plenty of other fun things to do! Have a blast!