I plan on being in Mykonos for 4 days. I am thinking about just taking a day trip to Santorini. Is that fine including attractions and ferry ride? I am not sure if it is worth it.. How much is the ferry ride ?

Thanks

With ferries it´s not possible. You will have to find an organized tour. But I think you wouldn´t. The first ferries to Santorini are around 14:30, arriving in Santorini at 17:45.
G.

Travel to Santorini from Mykonos requires an overnight.

And even two, if you will see the island, as the last boat to Mykonos leaves around 12:00

Two nights would be more realistic. FlyingCat 4 leaves Mykonos at 1445, arrives Santorini at 1725, and departs for Mykonos at 1150 the next morning. That's hardly enough time to do much more than check in and out of a hotel.

The only other ferry offering a direct connection between the two islands has a similar timetable.