Great job Flowerpower of showing the 2 challenges but I'm pretty sure the giant tall platform challenge will be for Immunity and the obstacle/puzzle challenge will be for Reward ("game-changing reward ")

In San Juan del Sur the Final 4 Immunity Challenge was this similar looking challenge:

We couldn't see an Immunity Necklace stand anywhere but it turned out to just be hanging on a thin stick strapped to the platform rails so I think it will be hard to see the Immunity Necklace.

There are a few more challenge photos that help to visualize the entire challenge and let's put them in order:

I think this is the Final 4 Reward Challenge:

Colors:

Turquoise: Tai
Red: Cydney
Yellow: Aubry
Blue: Michele

First crawl under a net

Then slide puzzle tiles off the end of table hitting a gong(?) and dropping into a box

See end of the slide table from the previous photo in this photo
After slide table navigate across Teeter Totter (Tai)
Then across the balance beam (Cydney)

Then there is a puzzle table that has six squares to put the correct 6 numbers into. OUCH Cydney! Aubry appears to be way ahead of everyone!

Finally dial in the correct 6 numbers and remove a bar releasing your flag and probably win an Advantage in the next Immunity Challenge or the power to remove a Juror!

And here I believe is the Final 4 Immunity Challenge

Colors:

Michele: Orange
Cydney: Blue
Tai: Yellow
Aubry: Red

All 4 start at the base of the structure and run out into the water to retrieve puzzle pieces

Jeff yells GO as they race into the ocean

All 4 return with the puzzle pieces and climb the tower where you can see all 4 at the top with Jeff

All 4 are working on this puzzle