On a four day trip I think you're going to be very pressed for time to begin with, and I'm nto sure I'd bother with a spa treatment, but that's me.
If you want the best in the area, Canyon Ranch at Gaylord Palms is the ticket.
IMHO the spas run by Niki Bryan are average, and the level of amenities and services vs. cost have declined over the years to the point that I don't go there any longer.
I haven't been to the spa at the Dolphin, but have heard nothing but good things about it.
If you never go to spas, I'm sure you'll be pleased with SSR or the GF. But if you've experienced world class spas, you'll be disappointed--they are far from world class.
Anne