How to Travel from Split to Ljubljana for 4 adults

We used Day Trip transfer service. Excellent drivers; clean, comfortable vehicles, door to door pick up and drop off; great customer service, on time drivers. Pricing increases the closer you get to departure, so book early, and then cancel if necessary. Their cancellation policy is more than generous. https://daytrip.com/

There are ~long duration buses and/or trains from Split to Zagreb, and then connecting
on to Ljubljana.

Flixbus is safe and reliable. Croatian Railways sells the tickets between Split and Zagreb.
For some reason Austrian Railways sells the tickets between Zagreb
and Ljubljana.

You would be potentially going right by places like Plitvice Lakes, which if you haven't
been there, seems a shame to miss. Renting a car and driving yourself would open up
those options. There would probably be a dropoff charge for Zagreb and a bigger one
going across borders into Slovenia. Maybe daytrip would also allow a stopover, but
Plitvice Lakes is usually a 4-6 hour visit.

I really do not recommend a day-time stopover at Plitvice. The park is beautiful, but it gets utterly swamped by daytrippers. The walkways (most of which have no railings) are absolutely packed, so you'll find yourself walking in lockstep with a mob. The only way to have a the sort of experience there one would anticipate is to spend a night in the area, ideally seeing part of the park late on your arrival day and the rest of the park early the next morning.

In one day? What is your budget?
I would insert a stopover in Zagreb if possible, as public is so easy for Split-Zagreb and Zag-Lju, but it's a very long day otherwise.

I agree with both the idea of staying a night in Zagreb as well as in Plitvice. But since OP has
not indicated how much time they have to expend on the transportation, I suppose a day trip
would be better than nothing. But if you had 1-2 nights, I'd say something like:

• go from Split to Plitvice (2-3 hours) morning
• See Plitvice starting mid-afternoon and plan on staying until close
• spend night in Plitvice
• go to Zagreb next morning (2 hrs) and spend afternoon seeing Zagreb
• either get to Ljubljana that evening (probably not in time for dinner) or spend night in Zagreb and get to Ljub the next morning.

In Croatia we used Octopus Transfers three times, and we were very happy with them. https://www.octopustransferscroatia.com/