Rewind

The land-untap clause is the whole design. Counter target spell at four mana is generic Counterspell math with a tax bolted on, the price of trading at instant speed; the untap-up-to-four-lands rider erases that tax. Cast it on an opponent's turn off four untapped lands and you have countered for free, all your mana refunded to leave up the next answer or chain into a second spell. That refund converts countermagic from a resource you spend into one you cycle, and it makes Rewind a conceptual ancestor of every "free counter that returns the mana" Wizards has printed since. The constraint is purely a matter of available mana. The rider only pays out if you have four lands to untap when you cast it, easiest on the opponent's turn while sitting behind open mana, though the clause is never truly dead: cast it on your own turn to win a counter war and you still untap four lands to spend in a current or second main phase. The real floor is the four-mana commitment. You cannot hold up Rewind and tap out the same turn, so a deck deploying threats on its own clock pays full price for a counter whose refund it rarely collects. A free counter for dedicated control, a clunky one for anyone with proactive plans: that split sits at the heart of the design, and it explains why Rewind has always looked sharper in theory than it feels across a table.