If this is the last door to the BBEG, then perhaps a culmination of sorts...the party must use the previous 3 elements to eliminate the earth statue...but the statue, after all of its transformations, endures.
Use wind on the stone will "erode" the statue into a sandy form....it keeps its shape, but is obviously a soft, granular substance as opposed to solid stone.
Use water on the "sandy" statue to turn it into a "clay-ish" substance. Again, the statue maintains it's shape...but the basic structure of the statue is a different "earth" than it started.
Use fire on it to "bake" the clay. It because kind of glossy and hardened and easily shatters into pieces...the pieces are still "earth"...they endure. Once the baked clay statue is destroyed, the elemental portal closes and their way becomes clear. (or to borrow from above, I do like the idea of the ruby -or some other gemstone-they're part of the earth too...diamond perhaps...diamonds are forever after all
So you could have the baked clay statue simply fall apart after it is baked...revealing the gemstone key.
That closer to what you were looking for? If not, hope it at least sparks some other concept for ya.
Have fun and happy sculpting.
--Steel Dragons