Can {Rewind} be cast without a target? And then still untap four lands??
You can only play a spell that 'targets' on a target. Any time a spell that says 'target' and there is none it fizzles. All parts of the spell fizzle. No untap
RESOLVED
Friend tried to play {Diluvian Primordial} then target the {Rewind} in my graveyard to untapped his four lands. He did not have anything to target {Rewind} with.
Remind him that those in the graveyard are just cards, not spells.
He said rewind was being cast from the grave by the primordial.
Scar, my bad.
I found a ruling belonging to {Diluvian Primordial}
1/24/2013 If you can't cast one of the target instant or sorcery cards, perhaps because there are no legal targets available, or if you choose not to cast one, it will remain in its owner's graveyard.
So I would think the rewind would stay in the graveyard and not casted. So yes, he can target rewind, but it does nothing since it has no legal targets.
To simplify the answers, no.