four-wheeler as in ATV/quad? Then yes, that.

WYR meet Bilbo or Sam?

*sigh* Frodo. Just kidding. 😀 Sam, I guess. 😀

WYR meet Edmund or Frodo?

Edmund!

WYR be on the wrong end of Susan's bow or Lucy's dagger?

Depends if it's book-verse or movie-verse. But I might be safer with Lucy's dagger.

WYR watch a sunrise at the beach or on a mountain?

(@Ariel: Why Edmund? Frodo is way too awesome!)

Mountain!!!! 😀

WYR play a clarinet or a flute?

(@Gyps: because Edmund is way more epic and my loyalties are to Narnia first. If the choice was between Edmund (Narnian third-favourite) and Aragorn (LotR first-favourite), I'd still go with Edmund even though Aragorn is also way epic.)

clarinet

WYR discover a way to live on Mars or fly to the reaches of the solar system?

Fly to the reaches of the solar system.

WYR have a star or a planet named after you?

Star!!! 😀 So cool. 😀

WYR be named Tatianna or Iyanna? 😀

Lyanna ("God has answered")

WYR try an exotic fruit or a (non-pasta) noodle dish?

Um, an exotic fruit, I suppose.

WYR play tag on big round haybales or play duck, duck, goose on a pile of little square haybales?

Tag! *needs hay bales first, though*

WYR be a chipmunk or a squirrel? 😛

Squirrel.

WYR shrink to a very tiny height or grow to an enormous height?

Grow to an enormous height.

WYR climb a mountain or explore a cave?

Explore a cave!! 😀

WYR explore a cave or explore some woods? 😀

Explore some woods!

WYR write and sing a song and hear it played on the radio or star in a movie and see the preview on TV?